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Nonergodicity in Electron Capture Dissociation Investigated Using Hydrated Ion Nanocalorimetry
Abstract
Hydrated divalent magnesium and calcium clusters are used as nanocalorimeters to measure the internal energy deposited into size-selected clusters upon capture of a thermally generated electron. The infrared radiation emitted from the cell and vacuum chamber surfaces as well as from the heated cathode results in some activation of these clusters, but this activation is minimal. No measurable excitation due to inelastic collisions occurs with the low-energy electrons used under these conditions. Two different dissociation pathways are observed for the divalent clusters that capture an electron: loss of water molecules (Pathway I) and loss of an H atom and water molecules (Pathway II). For Ca(H2O)
n
2+, Pathway I occurs exclusively for n ≥ 30 whereas Pathway II occurs exclusively for n ≤ 22 with a sharp transition in the branching ratio for these two processes that occurs for n ≈ 24. The number of water molecules lost by both pathways increases with increasing cluster size reaching a broad maximum between n = 23 and 32, and then decreases for larger clusters. From the number of water molecules that are lost from the reduced cluster, the average and maximum possible internal energy is determined to be ∼ and 5.2 eV, respectively, for Ca(H2O)
302+. This value is approximately the same as the calculated ionization energies of M(H2O)
n
+, M = Mg and Ca, for large n indicating that the vast majority of the recombination energy is partitioned into internal modes of the ion and that the dissociation of these ions is statistical. For smaller clusters, estimates of the dissociation energies for the loss of H and of water molecules are obtained from theory. For Mg(H2O)
n
2+, n = 4−6, the average internal energy deposition is estimated to be 4.2−4.6 eV. The maximum possible energy deposited into the n = 5 cluster is <7.1 eV, which is significantly less than the calculated recombination energy for this cluster. There does not appear to be a significant trend in the internal energy deposition with cluster size whereas the recombination energy is calculated to increase significantly for clusters with fewer than 10 water molecules. These, and other results, indicate that the dissociation of these smaller clusters is nonergodic.
... Precursor ions were isolated and after 50 ms, electrons were introduced into the ion cell from a heated cathode by pulsing the cathode housing to 1.5 V for 120 ms for the transition-metal containing nanodrops and to 1.4 V for 40 ms for the triply charged clusters. A detailed description of the experimental setup used here has been previously re- ported [23, 28] . Between 500 –1000 spectra were coadded to improve the signal-to-noise ratios in these experiments. ...
... In general, both smaller and larger clusters lose fewer water molecules than intermediate size clusters. For example, EC of Ca(H 2 O) n 2 results in the loss of 2.8, 10.2, and 9.0 water molecules for n 4, 32, and 47, respectively [23]. At smaller cluster sizes, fewer water molecules are lost despite high RE values because the binding energies of water molecules closest to the metal ions are higher and because more energy partitions into translational and rotational modes of the products due to the high initial effective temperatures of the reduced clusters. ...
... In addition to the extent of water molecule loss depending on cluster size, the competition between loss of exclusively water molecules (Pathway I) and the loss of a H atom and water molecules (Pathway II) upon EC by M(H 2 O) n z also depends on n. Pathway II is favored at small cluster sizes, whereas Pathway I is dominant at larger sizes, with a relatively rapid transition occurring between both pathways at n 24 and 44, for M Ca 2 [23], and M La 3 , respectively. Mn is the only divalent metal ion that dissociates by both pathways with 32 water molecules (Figure 1a ), although both pathways are observed for all the transition-metal ions with 24 water molecules [27]. ...
The recombination energies resulting from electron capture by a positive ion can be accurately measured using hydrated ion nanocalorimetry in which the internal energy deposition is obtained from the number of water molecules lost from the reduced cluster. The width of the product ion distribution in these experiments is predominantly attributable to the distribution of energy that partitions into the translational and rotational modes of the water molecules that are lost. These results are consistent with a singular value for the recombination energy. For large clusters, the width of the energy distribution is consistent with rapid energy partitioning into internal vibrational modes. For some smaller clusters with high recombination energies, the measured product ion distribution is narrower than that calculated with a statistical model. These results indicate that initial water molecule loss occurs on the time scale of, or faster than energy randomization. This could be due to inherently slow internal conversion or it could be due to a multi-step process, such as initial ion-electron pair formation followed by reduction of the ion in the cluster. These results provide additional evidence for the accuracy with which condensed phase thermochemical values can be deduced from gaseous nanocalorimetry experiments.
... Because of the wide range of SHE values obtained from previous indirect measurements and computational approaches, it is interesting to investigate more direct experimental methods to measure reduction potentials. We recently introduced a new gas-phase ion nanocalorimetry technique,2930313233343536 based on the ion thermometer method of Cooks and co-workers,[37] in which electrochemistry on large " aqueous " nanodrops is performed in vacuo to obtain absolute half-cell potentials in bulk solution.[29,30] Water nanodrops containing individual redox active Cu 2+ and [M(NH 3 ) 6 ] 3+ , for M = Ru, Co, Os, Cr, and Ir, can capture thermally generated electrons and results in loss of water molecules from the droplet.[29] ...
... In addition to the water molecule loss observed for larger clusters, electron capture (EC) by smaller hydrated metal ions can result in the loss of a hydrogen atom and water molecules. These two dissociation pathways are shown in Scheme 2. EC by Ca(H 2 O) n 2+ results in dissociation only by pathway II for n≤22, whereas only pathway I occurs for n≥30.[31] At n = 24, the abundance of each pathway is comparable and both pathways result in the loss of about ten water molecules from the reduced precursor. ...
... Because of the large number of water molecules lost, even small differences can be significant. Electronic structure calculations indicate that the sequential hydration energies for CaOH + are higher than for Ca + ;[31] for n = 1 and 5, the hydration energy of the hydroxide ion is 4.2 and 1.2 kcal mol −1 higher, respectively.[31] If the hydroxide-ioncontaining clusters have systematically higher binding energies than the M + -containing clusters, this would result in E(II) values that are systematically low; only a ≈0.4 kcal mol −1 higher water molecule binding energy to MOH + versus M + containing clusters could account for the 0.16 V difference in the absolute SHE potential obtained by these two nanocalorimetry methods. ...
Hydrated ion nanocalorimetry is used to measure reduction energies and H atom affinities of gaseous hydrated ions by determining the energy deposited into these nanodrops from the number of water molecules lost upon reduction by thermally generated electrons (see figure).
Solution-phase, half-cell potentials are measured relative to other half-cell potentials, resulting in a thermochemical ladder that is anchored to the standard hydrogen electrode (SHE), which is assigned an arbitrary value of 0 V. A new method for measuring the absolute SHE potential is demonstrated in which gaseous nanodrops containing divalent alkaline-earth or transition-metal ions are reduced by thermally generated electrons. Energies for the reactions 1) M(H2O)242+(g)+e−(g)→M(H2O)24+(g) and 2) M(H2O)242+(g)+e−(g)→MOH(H2O)23+(g)+H(g) and the hydrogen atom affinities of MOH(H2O)23+(g) are obtained from the number of water molecules lost through each pathway. From these measurements on clusters containing nine different metal ions and known thermochemical values that include solution hydrolysis energies, an average absolute SHE potential of +4.29 V vs. e−(g) (standard deviation of 0.02 V) and a real proton solvation free energy of −265 kcal mol−1 are obtained. With this method, the absolute SHE potential can be obtained from a one-electron reduction of nanodrops containing divalent ions that are not observed to undergo one-electron reduction in aqueous solution.
... In this ion nanocalorimetry method, gas-phase nanodrops that contain charged metal ions are reduced by thermally generated electrons (28-31). For large clusters, the RE is deposited into internal modes of the reduced cluster resulting in cluster heating and evaporation of water molecules (28,29). An estimate of the RE can be obtained from the sum of the sequential threshold dissociation energies for the maximum number of water molecules that are lost. ...
... An alternative approach to investigating interactions between ions, electrons, and water is to measure recombination energies (REs) resulting from electron capture (EC) by nanometer-sized hydrated ions in the gas phase (28). In this ion nanocalorimetry method, gas-phase nanodrops that contain charged metal ions are reduced by thermally generated electrons (28)(29)(30)(31). For large clusters, the RE is deposited into internal modes of the reduced cluster resulting in cluster heating and evaporation of water molecules (28,29). ...
... In this ion nanocalorimetry method, gas-phase nanodrops that contain charged metal ions are reduced by thermally generated electrons (28)(29)(30)(31). For large clusters, the RE is deposited into internal modes of the reduced cluster resulting in cluster heating and evaporation of water molecules (28,29). An estimate of the RE can be obtained from the sum of the sequential threshold dissociation energies for the maximum number of water molecules that are lost. ...
A gaseous nanocalorimetry approach is used to investigate effects of hydration and ion identity on the energy resulting from ion–electron recombination. Capture of a thermally generated electron by a hydrated multivalent ion results in either loss of a H atom accompanied by water loss or exclusively loss of water. The energy resulting from electron capture by the precursor is obtained from the extent of water loss. Results for large-size-selected clusters of Co(NH3)6(H2O)n3⁺ and Cu(H2O)n2⁺ indicate that the ion in the cluster is reduced on electron capture. The trend in the data for Co(NH3)6(H2O)n3⁺ over the largest sizes (n ≥ 50) can be fit to that predicted by the Born solvation model. This agreement indicates that the decrease in water loss for these larger clusters is predominantly due to ion solvation that can be accounted for by using a model with bulk properties. In contrast, results for Ca(H2O)n2⁺ indicate that an ion–electron pair is formed when clusters with more than ≈20 water molecules are reduced. For clusters with n = ≈20–47, these results suggest that the electron is located near the surface, but a structural transition to a more highly solvated electron is indicated for n = 47–62 by the constant recombination energy. These results suggest that an estimate of the adiabatic electron affinity of water could be obtained from measurements of even larger clusters in which an electron is fully solvated.
• clusters
• ECD
• recombination
• hydration
... 37,38 An alternative approach to measure internal energy deposited by electron capture, or any other activation method, uses solvated ions as nanocalorimeters. 32,[44][45][46][47][48] This method has been demonstrated with hydrated di-and trivalent ions and has been used to measure the internal energy deposition in ECD experiments as a function of the cluster size and metal ion identity. In brief, activation of a hydrated ion results in evaporation of water molecules from the cluster. ...
... The formation of Ca + from the larger reduced clusters is surprising because of the large internal energy deposition necessary to evaporate all the water molecules from the cluster and because the competitive formation of CaOH + is energetically favored at small cluster sizes. 32 The appearance of Ca + in these experiments indicates that loss of water molecules is kinetically favored over loss of a H atom at the smaller cluster sizes. ...
Ion nanocalorimetry is used to investigate the internal energy deposited into M (2+)(H 2O) n , M = Mg ( n = 3-11) and Ca ( n = 3-33), upon 100 keV collisions with a Cs or Ne atom target gas. Dissociation occurs by loss of water molecules from the precursor (charge retention) or by capture of an electron to form a reduced precursor (charge reduction) that can dissociate either by loss of a H atom accompanied by water molecule loss or by exclusively loss of water molecules. Formation of bare CaOH (+) and Ca (+) by these two respective dissociation pathways occurs for clusters with n up to 33 and 17, respectively. From the threshold dissociation energies for the loss of water molecules from the reduced clusters, obtained from binding energies calculated using a discrete implementation of the Thomson liquid drop model and from quantum chemistry, estimates of the internal energy deposition can be obtained. These values can be used to establish a lower limit to the maximum and average energy deposition. Not taking into account effects of a kinetic shift, over 16 eV can be deposited into Ca (2+)(H 2O) 33, the minimum energy necessary to form bare CaOH (+) from the reduced precursor. The electron capture efficiency is at least a factor of 40 greater for collisions of Ca (2+)(H 2O) 9 with Cs than with Ne, reflecting the lower ionization energy of Cs (3.9 eV) compared to Ne (21.6 eV). The branching ratio of the two electron capture dissociation pathways differs significantly for these two target gases, but the distributions of water molecules lost from the reduced precursors are similar. These results suggest that the ionization energy of the target gas has a large effect on the electron capture efficiency, but relatively little effect on the internal energy deposited into the ion. However, the different branching ratios suggest that different electronic excited states may be accessed in the reduced precursor upon collisions with these two different target gases.
... Based on the evidence and discussion above, we propose that nonstatistical partitioning of energy into the neutralized monoanion is a significant process during CAPTR experiments, which would result in significantly less heating of charge-reduced protein ions than suggested by the large total change in free energy associated with each CAPTR event. Although some interpretations of ECD and ETD data invoke nonstatistical partitioning of energy after the polycation combines with an electron (Breuker et al., 2004;Leib et al., 2007), all models are consistent with the bulk of that recombination energy being available to the reduced cation (Syrstad & Turecček, 2005;Turecček et al., 2008;Tureček & Julian, 2013). Note that in ECD, a free electron combines with a polycation; without fragmentation the entire recombination energy must partition into the protein. ...
Collision cross‐section values, which can be determined using ion mobility experiments, are sensitive to the structures of protein ions and useful for applications to structural biology and biophysics. Protein ions with different charge states can exhibit very different collision cross‐section values, but a comprehensive understanding of this relationship remains elusive. Here, we review cation‐to‐anion, proton‐transfer reactions (CAPTR), a method for generating a series of charge‐reduced protein cations by reacting quadrupole‐selected cations with even‐electron monoanions. The resulting CAPTR products are analyzed using a combination of ion mobility, mass spectrometry, and collisional activation. We compare CAPTR to other charge‐manipulation strategies and review the results of various CAPTR‐based experiments, exploring their contribution to a deeper understanding of the relationship between protein ion structure and charge state.
... I showed radial probability distributions only for Rydberg orbitals as high as 6s because my study 24 showed that the crosssections for electrons (or donor anions) interacting with Rydberg orbitals are comparable for n = 3-6 but decrease considerably for higher n values. The overlap of the electron's wave function with the Rydberg orbital decreases with increasing n as suggested in Fig. 2, but the πr 2 factor in the cross-section increases with n with the two factors combining to produce the comparable values for the n = 3 to n = 6 cross-sections. ...
Within any molecule or cluster containing one or more positively charged sites, families of Rydberg orbitals exist. Free electrons can attach directly, and anionic reagents with low electron binding energy can transfer an electron into one of these orbitals to form a neutral Rydberg radical. The possibilities that such a radical could form a covalent bond either to another Rydberg radical or to a radical holding its electron in a conventional valence orbital are considered. This Perspective overviews two roles that Rydberg radicals can play, both of which have important chemical consequences. Attachment of an electron into excited Rydberg orbitals is followed by rapid (∼10-6 s) relaxation into the lowest-energy Rydberg orbital to form the ground state radical. Although the excited Rydberg species are stable with respect to fragmentation, the ground-state species is usually quite fragile and undergoes homolytic bond cleavage (e.g., -R2NH dissociates into -R2N + H or into -RNH + R) by overcoming a very small barrier on its potential energy surface, thus generating reactive radicals (H or R). Here, it is shown that as a result of this fragility, any covalent bonds formed by Rydberg radicals are weak and the molecules they form are susceptible to exothermic fragmentations that involve quite small activation barriers. Another role played by Rydberg species arises when the Coulomb potentials provided by the (one or more) positive site(s) in the molecule stabilize low-energy anti-bonding orbitals (e.g., σ* orbitals of weak σ bonds or low-lying π* orbitals) to the extent that electron attachment into these Coulomb-stabilized orbitals is rendered exothermic. In such cases, the overlap of the Rydberg orbitals on the positive site(s) with the σ* or π* orbitals allows either a free electron or a weakly bound electron to an anionic reagent that is attracted toward the positive site by its Coulomb force to be guided/transferred into the σ* or π* orbital instead. After attaching to such an anti-bonding orbital, bond cleavage occurs again, generating reactive radical species. Because of the large radial extent of Rydberg orbitals, this class of bond cleavage events can occur quite distant from the positively charged group. In this Perspective, several examples of both types of phenomena are given for illustrative purposes.
... The ergodicity or non-ergodicity mechanism of ECD is still hotly debated. 22,23 McLafferty and co-workers proposed the Cornell mechanism (Scheme S1a), 18 where electron attachment occurs in a Rydberg orbital at a basic site (e.g., N-terminus, Arg, Lys or His residues) leading to a hypervalent radical species. The nature of this orbital combined with the excess energy supplied by the electron (4 to 6 eV) allow to reach relaxation processes that result in H transfer leading to the cleavage of the peptide backbone N-Cα bond (Scheme S1a). ...
In the present work, four, well-studied, model peptides (e.g., substance P, bradykinin, angiotensin I and AT-Hook 3) were used to correlate structural information provided by ion mobility and ECD/CID fragmentation in a TIMS-q-EMS-ToF MS/MS platform, incorporporating an electromagnetostatic cell (EMS). The structural heterogeneity of the model peptides was observed by (i) multi-component ion mobility profiles (high ion mobility resolving power, R ∼115-145), and (ii) fast online characteristic ECD fragmentation patterns per ion mobility band (∼0.2 min). Particularly, it was demonstrated that all investigated species were probably conformers, involving cis/trans-isomerizations at X-Pro peptide bond, following the same protonation schemes, in good agreement with previous ion mobility and single point mutation experiments. The comparison between ion mobility selected ECD spectra and traditional FT-ICR ECD MS/MS spectra showed comparable ECD fragmentation efficiencies but differences in the ratio of radical (˙)/prime (') fragment species (H˙ transfer), which were associated with the differences in detection time after the electron capture event. The analysis of model peptides using online TIMS-q-EMSToF MS/MS provided complementary structural information on the intramolecular interactions that stabilize the different gas-phase conformations to those obtained by ion mobility or ECD alone.
... It should be mentioned that the addition of more than six ammonia ligands is expected to stabilize the hexa-coordinated metal ammonia entities as happens for magnesium and calcium water complexes. [28][29][30][31] Therefore, they are expected to be present in metal ammonia solutions. ...
High level quantum chemical approaches are used to study the geometric and electronic structures of M(NH3)n and M(NH3)n⁺ (M = Cr, Mo for n = 1–6). These complexes possess a dual shell electronic structure of the inner metal (3d or 4d) orbitals and the outer diffuse orbitals surrounding the periphery of the complex. Electronic excitations reveal these two shells to be virtually independent of the other. Molybdenum and chromium ammonia complexes are found to differ significantly in geometry with the former adopting an octahedral geometry and the latter a Jahn–Teller distorted octahedral structure where only the axial distortion is stable. The hexa-coordinated complexes and the tetra-coordinated complexes with two ammonia molecules in the second solvation shell are found to be energetically competitive. Electronic excitation energies and computed IR spectra are provided to allow the two isomers to be experimentally distinguished. This work is a component of an ongoing effort to study the periodic trends of transition metal solvated electron precursors.
... For instance, experimental and theoretical evidence have shown that the presence of solvent molecules on either free or supported gold clusters enhances the catalytic activity for the O 2 dissociation, resulting in facilitating the CO oxidation. 6−16 In the gas-phase experiments, a number of solvated metal ions have been detected through collision-induced dissociation (CID), 17,18 guided ion-beam mass spectrometry (GIBMS), 19 ion/neutral chemistry, 20 high-pressure mass spectrometry (HPMS), 21,22 electron-capture dissociation (ECD), 18,23,24 infrared photodissociation (IRPD) spectroscopy, 1,2 photoelectron spectroscopy (PES), 25−27 etc. In particular, IRPD and PES have been widely used to characterize the structures of the cluster ions. ...
We firstly demonstrate the photoelectron spectroscopic evidence of the transition of two competitive solvation patterns in the Au─(CH3OH)n (n = 1-5) clusters. Quantum chemical calculations have been carried out to characterize the geometric structures, energy properties and hydrogen-bonded patterns, and to aid the spectral assignment. It has been found that the interior solvated state is favored in the small clusters at n = 1 and 2, whereas the surface solvated state becomes favored in the larger clusters at n = 4 and 5. Both solvated states are competitive at n = 3, revealing a transition point here. Such trend is supported by the calculated binding energies. This finding suggests that the Au─ anion is likely to be solvated on the surface in the even larger clusters.
... Recent ion nanocalorimetry experiments have been used to measure the RE when an aqueous nanodrop containing a metal ion is reduced by a thermally generated electron in the gas phase. [44][45][46][47][48] In brief, when an electron is captured by a charged nanodrop, the RE can be deposited into internal modes of the nanodrop resulting in the evaporation of water molecules. The value of the RE can be obtained from the sum of the threshold dissociation energies for the maximum number of water molecules that are lost from the cluster. ...
The effects of water on electron capture dissociation products, molecular survival, and recombination energy are investigated for diprotonated Lys-Tyr-Lys solvated by between zero and 25 water molecules. For peptide ions with between 12 and 25 water molecules attached, electron capture results in a narrow distribution of product ions corresponding to primarily the loss of 10-12 water molecules from the reduced precursor. From these data, the recombination energy (RE) is determined to be equal to the energy that is lost by evaporating on average 10.7 water molecules, or 4.3 eV. Because water stabilizes ions, this value is a lower limit to the RE of the unsolvated ion, but it indicates that the majority of the available RE is deposited into internal modes of the peptide ion. Plotting the fragment ion abundances for ions formed from precursors with fewer than 11 water molecules as a function of hydration extent results in an energy resolved breakdown curve from which the appearance energies of the b 2 (+), y 2 (+), z 2 (+*), c 2 (+), and (KYK + H) (+) fragment ions formed from this peptide ion can be obtained; these values are 78, 88, 42, 11, and 9 kcal/mol, respectively. The propensity for H atom loss and ammonia loss from the precursor changes dramatically with the extent of hydration, and this change in reactivity can be directly attributed to a "caging" effect by the water molecules. These are the first experimental measurements of the RE and appearance energies of fragment ions due to electron capture dissociation of a multiply charged peptide. This novel ion nanocalorimetry technique can be applied more generally to other exothermic reactions that are not readily accessible to investigation by more conventional thermochemical methods.
... Recently, we demonstrated that hydrated cluster ions are ideal "nanocalorimeters" that can be used to measure accurately the internal energy deposited into these ions upon activation. [24][25][26][27] This nanocalorimetry method has been used to measure the internal energy deposited by EC with thermally generated electrons as a function of cluster size and cation identity. 24 26 For these large clusters, all the available recombination energy is deposited into internal modes of the ion and the dissociation is statistical. ...
Ion nanocalorimetry is used to measure the effects of electron kinetic energy in electron capture dissociation (ECD). With ion nanocalorimetry, the internal energy deposited into a hydrated cluster upon activation can be determined from the number of water molecules that evaporate. Varying the heated cathode potential from -1.3 to -2.0 V during ECD has no effect on the average number of water molecules lost from the reduced clusters of either [Ca(H2O)15]2+ or [Ca(H2O)32]2+, even when these data are extrapolated to a cathode potential of zero volts. These results indicate that the initial electron kinetic energy does not go into internal energy in these ions upon ECD. No effects of ion heating from inelastic ion-electron collisions are observed for electron irradiation times up to 200 ms, although some heating occurs for [Ca(H2O)17]2+ at longer irradiation times. In contrast, this effect is negligible for [Ca(H2O)32]2+, a cluster size typically used in nanocalorimetry experiments, indicating that energy transfer from inelastic ion-electron collisions is negligible compared with effects of radiative absorption and emission for these larger clusters. These results have significance toward establishing the accuracy with which electrochemical redox potentials, measured on an absolute basis in the gas phase using ion nanocalorimetry, can be related to relative potentials measured in solution.
A molecular anion's (MA's) chemical reactivity and physical behavior can be quite different when it is surrounded by other molecules than when it exists in isolation. This sensitivity to the surrounding environment is especially high for anions because their outermost valence electrons are typically loosely bound and exist in rather spatially diffuse orbitals, allowing even weak intermolecular interactions arising from the environment to have strong effects. This Perspective offers illustrations of such sensitivity for a variety of cases including (i) the effect of solvation on electron binding energies, (ii) how some "well known" anions need to have solvent molecules around to even exist as stable species, (iii) how internal Coulomb repulsions within a multiply charged MA can provide temporary stability toward electron loss, (iv) how MAs arrange themselves spatially near liquid/vapor interfaces in manners that can produce unusual reactivity, (v) how nearby cationic sites can facilitate electron attachment to form a MA site elsewhere, (vi) how internal vibrational or rotational energy can make a MA detach an electron.
The formation of ternary aqua complexes of metal-based diagnostics and therapeutics is closely correlated to their in vivo efficacy but approaches to quantify the presence of coordinated water ligands are limited. We introduce a general and high-throughput method for characterizing the hydration state of para- and diamagnetic coordination complexes in the gas phase based on variable-temperature ion trap tandem mass spectrometry. Ternary aqua complexes are directly observed in the mass spectrum and quantified as a function of ion trap temperature. We recover expected periodic trends for hydration across the lanthanides and distinguish complexes with several inner-sphere water ligands by inspection of temperature-dependent speciation curves. We derive gas-phase thermodynamic parameters for discernable inner and second-sphere hydration events, and discuss their application to predict solution-phase behavior. The differences in temperature at which water binds in the inner and outer spheres arise primarily from entropic effects. The broad applicability of this method allows us to estimate the hydration states of Ga, Sc and Zr complexes under active preclinical and clinical study with as-yet undetermined hydration number. Variable-temperature mass spectrometry emerges as a general tool to characterize and quantitate trends in inner-sphere hydration across the periodic table.
Reductions of O2, CO2, and CH3CN by the half-reaction of the Mg(II)/Mg(I) couple (Mg(2+) + e(-) → Mg(+•)) confined in a nano-sized water droplet ([Mg(H2O)16](•+)) have been examined theoretically by means of density functional theory based molecular dynamics methods. The present works have revealed many intriguing aspects of the reaction dynamics of the water clusters within several picoseconds or even in sub-picoseconds. The reduction of O2 requires an overall doublet spin state of the system. The reductions of CO2 and CH3CN are facilitated by their bending vibrations and the electron-transfer processes complete within 0.5 ps. For all reactions studied, the radical anions, i.e. O2(•-), CO2(•-), and CH3CN(•-), are initially formed on the cluster surface. O2(•-) and CO2(•-) can integrate into the clusters due to their high hydrophilicity. They are either solvated in the second solvation shell of Mg(2+) as a solvent-separated ion pair (ssip) or directly coordinated to Mg(2+) as a contact-ion pair (cip) having the (1)η-[MgO2](•+) and (1)η-[MgOCO](•+) coordination modes. The (1)η-[MgO2](•+) core is more crowded than the (1)η-[MgOCO](•+) core. The reaction enthalpies of the formation of ssip and cip of [Mg(CO2)(H2O)16](•+) are -36 ± 4 kJ mol(-1) and -30 ± 9 kJ mol(-1), respectively, which were estimated based on the average temperature changes during the ion-molecule reaction between CO2 and [Mg(H2O)16](•+). The values for the formation of ssip and cip of [Mg(O2)(H2O)16](•+) are estimated to be -112 ± 18 kJ mol(-1) and -128 ± 28 kJ mol(-1), respectively. CH3CN(•-) undergoes protonation spontaneously to form the hydrophobic [CH3CN, H](•). Both CH3CN and [CH3CN, H](•) cannot efficiently penetrate into the clusters with activation barriers of 22 kJ mol(-1) and ~40 kJ mol(-1), respectively. These results provide fundamental insights into the solvation dynamics of the Mg(2+)/Mg(•+) couple on the molecular level.
In solution, half-cell potentials and ion solvation energies (or enthalpies) are measured relative to other values, thus establishing ladders of thermochemical values that are referenced to the potential of the standard hydrogen electrode (SHE) and the proton hydration energy (or enthalpy), respectively, which are both arbitrarily assigned a value of 0. In this focused review article, we describe three routes for obtaining absolute solution-phase half-cell potentials using ion nanocalorimetry, in which the energy resulting from electron capture (EC) by large hydrated ions in the gas phase are obtained from the number of water molecules lost from the reduced precursor cluster, which was developed by the Williams group at the University of California, Berkeley. Recent ion nanocalorimetry methods for investigating ion and electron hydration and for obtaining the absolute hydration enthalpy of the electron are discussed. From these methods, an absolute electrochemical scale and ion solvation scale can be established from experimental measurements without any models.
The reactions of gas-phase hydrated electron (H2O)(n)(center dot-), n < 75, with acrylic acid, methyl acrylate and vinyl acetate monomers are studied by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. The first reaction step is uptake of the reactant molecule with formation of a hydrated radical anion. The subsequent reaction steps, however, are remarkably different. With acrylic acid, products of the composition (C2H3COOH)(m)(center dot-), m <= 6, are observed, which are evidence for oligomerization. With methyl acrylate, uptake of a second molecule is followed by elimination of methanol, which is rationalized with the formation of a five-membered ring. Formation of hydrated hydroxide OH-(H2O)(n) happens to a small extent, which is evidence for the reduction of methyl acrylate to C2H4COOCH3 center dot. Only one molecule of vinyl acetate is taken up by the clusters. After longer reaction delays, cleavage of the molecule with formation of C2H3O-(H2O)(n) is observed, which occurs preferentially at n approximate to 10. (c) 2012 Elsevier B.V. All rights reserved.
The electron capture dissociation (ECD) of [Mg(H2O)n]2+ clusters is examined using ab initio electronic structure methods to interpret experimental data on [Mg(H2O)n]2+ and [Ca(H2O)n]2+. Calculations are performed on Mg2+(H2O)n clusters containing a full first hydration shell plus one or two additional water molecules positioned to represent second- and third-shell molecules. The propensity of the Mg-containing clusters to undergo fragmentation primarily into [Mg(H2O)n−m]1++m H2O (m=10) for n>17 but primarily into [Mg(OH)(H2O)n−k]1++H+(k−1) H2O (k−1=10) for n
Potential energy curves and coupling matrix elements of the one-electron capture by Mg2+ ions in collision with Zn atoms have been determined at the multi-reference configuration interaction level of theory. The cross sections calculated using a semi-classical approach in the [1–300]keV laboratory energy range are in good agreement with the experimental measurements and reflect the importance of the molecular description.
Complexes of doubly protonated 1,n-diaminoalkanes with one or two molecules of 18-crown-6-ether undergo consecutive and competitive dissociations upon electron capture from a free thermal electron and femtosecond collisional electron transfer from Na and Cs atoms. The electron capture dissociation (ECD) and electron capture-induced dissociation (ECID) mass spectra show very different products and product ion intensities. In ECD, the reduced precursor ions dissociate primarily by loss of an ammonium hydrogen and the crown ether ligand. In ECID, ions from many more dissociation channels are observed and depend on whether collisions occur with Na or Cs atoms. ECID induces highly endothermic CC bond cleavages along the diaminoalkane chain, which are not observed with ECD. Adduction of one or two crown ethers to diaminoalkanes results in different electron capture cross-sections that follow different trends for ECD and ECID. Electron structure calculations at the B3-PMP2/6-311++G(2d,p) level of theory were used to determine structures of ions and ion radicals and the energetics for protonation, electron transfer, and ion dissociations for most species studied experimentally. The calculations revealed that the crown ether ligand substantially affected the recombination energy of the diaminoalkane ion and the electronic states accessed by electron attachment.
Here we review recent studies on the metastable fragmentation of individual DNA and RNA building blocks and their compositions using matrix assisted laser desorption/ionization mass spectrometry (MALDI). To compare the fragmentation channels of small DNA components with larger compositions we have studied the metastable fragmentation of the deprotonated nucleobases, ribose, ribose-monophoshates, the nucleosides, the nucleoside 5 -monophosphates and selected oligonucleotides. Both previously published and unpublished data are reported. To gain a comprehensive picture of the fragmentation of individual components, metastable fragmentation of native components are in many cases compared to chemically modified components and isotopic labelling is used to unambiguously identify fragments. Furthermore, to shed light on the underlying fragmentation mechanisms we complement the experimental studies with classical dynamics simulations of the fragmentation of selected compounds. For the DNA and RNA com-ponents where dissociative electron attachment studies have been conducted we compare these to the metastable fragmentation channels observed here.
The chemistry of (H(2)O)(n)(•-), CO(2)(•-)(H(2)O)(n), and O(2)(•-)(H(2)O)(n) with small sulfur-containing molecules was studied in the gas phase by Fourier transform ion cyclotron resonance mass spectrometry. With hydrated electrons and hydrated carbon dioxide radical anions, two reactions with relevance for biological radiation damage were observed, cleavage of the disulfide bond of CH(3)SSCH(3) and activation of the thiol group of CH(3)SH. No reactions were observed with CH(3)SCH(3). The hydrated superoxide radical anion, usually viewed as major source of oxidative stress, did not react with any of the compounds. Nanocalorimetry and quantum chemical calculations give a consistent picture of the reaction mechanism. The results indicate that the conversion of e(-) and CO(2)(•-) to O(2)(•-) deactivates highly reactive species and may actually reduce oxidative stress. For reactions of (H(2)O)(n)(•-) with CH(3)SH as well as CO(2)(•-)(H(2)O)(n) with CH(3)SSCH(3), the reaction products in the gas phase are different from those reported in the literature from pulse radiolysis studies. This observation is rationalized with the reduced cage effect in reactions of gas-phase clusters.
The transition states of a chemical reaction in solution are generally accessed through exchange of thermal energy between the solvent and the reactants. As such, an ensemble of reacting systems approaches the transition state configuration of reactant and surrounding solvent in an incoherent manner that does not lend itself to direct experimental observation. Here we describe how gas-phase cluster chemistry can provide a detailed picture of the microscopic mechanics at play when a network of six water molecules mediates the trapping of a highly reactive "hydrated electron" onto a neutral CO(2) molecule to form a radical anion. The exothermic reaction is triggered from a metastable intermediate by selective excitation of either the reactant CO(2) or the water network, which is evidenced by the evaporative decomposition of the product cluster. Ab initio molecular dynamics simulations of energized CO(2)·(H(2)O)(6)(-) clusters are used to elucidate the nature of the network deformations that mediate intracluster electron capture, thus revealing the detailed solvent fluctuations implicit in the Marcus theory for electron-transfer kinetics in solution.
The effects of water on ion fluorescence were investigated, and average sequential water molecule binding energies to hydrated ions, M(z)(H(2)O)(n), at large cluster size were measured using ion nanocalorimetry. Upon 248-nm excitation, nanodrops with ~25 or more water molecules that contain either rhodamine 590(+), rhodamine 640(+), or Ce(3+) emit a photon with average energies of approximately 548, 590, and 348 nm, respectively. These values are very close to the emission maxima of the corresponding ions in solution, indicating that the photophysical properties of these ions in the nanodrops approach those of the fully hydrated ions at relatively small cluster size. As occurs in solution, these ions in nanodrops with 8 or more water molecules fluoresce with a quantum yield of ~1. Ce(3+) containing nanodrops that also contain OH(-) fluoresce, whereas those with NO(3)(-) do not. This indirect fluorescence detection method has the advantages of high sensitivity, and both the size of the nanodrops as well as their constituents can be carefully controlled. For ions that do not fluoresce in solution, such as protonated tryptophan, full internal conversion of the absorbed 248-nm photon occurs, and the average sequential water molecule binding energies to the hydrated ions can be accurately obtained at large cluster sizes. The average sequential water molecule binding energies for TrpH(+)(H(2)O)(n) and a doubly protonated tripeptide, [KYK + 2H](2+)(H(2)O)(n), approach asymptotic values of ~9.3 (n ≥ 11) and ~10.0 kcal/mol (n ≥ 25), respectively, consistent with a liquidlike structure of water in these nanodrops.
Fragmentation of metastable anions of all deoxynucleosides and nucleosides constituting DNA and RNA has been studied experimentally and by computer simulations. The ions were formed through deprotonation in matrix assisted laser desorption/ionisation (MALDI). Clear difference in fragmentation patterns was obtained for nucleosides containing purine vs. pyrimidine bases. To elucidate the role of various potential deprotonation sites, systematic blocking by chemical modification was performed and this gave unambiguous correlation between deprotonation sites and fragments observed. Classical dynamics simulations of the fragmentation process, using density functional theory to describe the electronic degrees of freedom, were performed for the various deprotonation sites. These were found to reproduce the observed fragmentation patterns remarkably well.
The reactions of the isomers of di- and trifluorobenzene with hydrated electrons (H(2)O)(n)(-), n = 19-70, have been studied by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. While Birch reduction, i.e. H atom transfer to the aromatic ring, was observed for all studied isomers, a strong dependence on the substitution pattern was observed for fluorine abstraction. Nanocalorimetry combined with G3 calculations are used to analyze the thermochemistry of the reactions. Fluorine abstraction is at least 100 kJ mol(-1) more exothermic than Birch reduction, yet the latter is the dominant reaction pathway for all three difluorobenzene isomers. Fluorine abstraction and Birch reduction face activation barriers of comparable magnitude. The relative barrier height is sensitive to the substitution pattern. Birch reduction occurs selectively with 1,3- and 1,4-difluorobenzene in a nanoscale aqueous environment.
Singly and doubly charged scandium-water ion-molecule complexes are produced in a supersonic molecular beam by laser vaporization. These ions are mass analyzed and size selected in a specially designed reflectron time-of-flight spectrometer. To probe their structure, vibrational spectroscopy is measured for these complexes in the O-H stretching region using infrared laser photodissociation and the method of rare gas atom predissociation, also known as "tagging." The O-H stretches in these systems are shifted to lower frequency than those for the free water molecule, and the intensity of the symmetric stretch band is strongly enhanced relative to the asymmetric stretch. These effects are more prominent for the doubly charged ions. Partially resolved rotational structure for the Sc(+)(H(2)O)Ar complex shows that the H-O-H bond angle is larger than it is in the free water molecule. Fragmentation and spectral patterns indicate that the coordination of the Sc(2+) ion is filled with six ligands (one water and five argons).
The average sequential water molecule binding enthalpies to large water clusters (between 19 and 124 water molecules) containing divalent ions were obtained by measuring the average number of water molecules lost upon absorption of an UV photon (193 or 248 nm) and using a statistical model to account for the energy released into translations, rotations, and vibrations of the products. These values agree well with the trend established by more conventional methods for obtaining sequential binding enthalpies to much smaller hydrated divalent ions. The average binding enthalpies decrease to a value of ~10.4 kcal/mol for n > ~40 and are insensitive to the ion identity at large cluster size. This value is close to that of the bulk heat of vaporization of water (10.6 kcal/mol) and indicates that the structure of water in these clusters may more closely resemble that of bulk liquid water than ice, owing either to a freezing point depression or rapid evaporative cooling and kinetic trapping of the initial liquid droplet. A discrete implementation of the Thomson equation using parameters for liquid water at 0 °C generally fits the trend in these data but provides values that are ~0.5 kcal/mol too low.
An improved cluster pair correlation method that is based on the method originally introduced by Tuttle et al. ( Tuttle et al. J. Phys. Chem. A 2002 , 106 , 925 - 932 ) was developed and evaluated using a significantly larger data set than used previously. With this larger data set, values for the absolute proton hydration free energy of -259.3 and -265.0 kcal/mol were obtained using the original and improved method, respectively. The former value is ∼4.5 kcal/mol less negative than previously reported values obtained with the same method but with smaller data sets. The dependence of this value on data set size indicates that the uncertainty in the original method may be greater than previously realized. The improved method has the advantages of higher precision, and the effects of cluster size on the proton hydration free energy and enthalpy values can be more readily evaluated. Data for ions with extreme pK(a)s, many of which were included in previous estimates of the proton hydration free energy, were found to be unreliable and were eliminated from the extended data set. There is only a subtle effect of cluster size on the Gibbs free energy values, and within the limits of the approximation inherent in the cluster pair correlation method, the "best" value for the standard absolute proton hydration free energy obtained with this new method and larger data set is -263.4 kcal/mol (average for clusters with 4-6 water molecules). The absolute proton hydration enthalpy values decrease from -273.1 to -275.3 kcal/mol with increasing cluster size (one to six water molecules, respectively). This trend, along with an anomalously high value for the absolute proton hydration entropy, indicates that the enthalpy obtained with this method may not have converged for these relatively small clusters.
The internal energy deposited in both on- and off-resonance collisional activation in Fourier transform ion cyclotron resonance mass spectrometry is measured with ion nanocalorimetry and is used to obtain information about the dissociation energy and entropy of a protonated peptide. Activation of Na(+)(H(2)O)(30) results in sequential loss of water molecules, and the internal energy of the activated ion can be obtained from the abundances of the product ions. Information about internal energy deposition in on-resonance collisional activation of protonated peptides is inferred from dissociation data obtained under identical conditions for hydrated ions that have similar m/z and degrees-of-freedom. From experimental internal energy deposition curves and Rice-Ramsperger-Kassel-Marcus (RRKM) theory, dissociation data as a function of collision energy for protonated leucine enkephalin, which has a comparable m/z and degrees-of-freedom as Na(+)(H(2)O)(30), are modeled. The threshold dissociation energies and entropies are correlated for data acquired at a single time point, resulting in a relatively wide range of threshold dissociation energies (1.1 to 1.7 eV) that can fit these data. However, this range of values could be significantly reduced by fitting data acquired at different dissociation times. By measuring the internal energy of an activated ion, the number of fitting parameters necessary to obtain information about the dissociation parameters by modeling these data is reduced and could result in improved accuracy for such methods.
A new variant of nanocalorimetry is proposed for the thermochemical analysis of ion-molecule reactions of hydrated ions in the gas phase. The average number of water molecules evaporating during the reaction is extracted by quantitative modeling of the average number of water molecules in the reactant and product cluster distribution as a function of time, taking into account black-body radiation induced dissociation. The method is tested on reactions of (H2O)n(-) with O2 and CO2, and the core exchange reaction of CO2(-)(H2O)n with O2 to yield O2(-)(H2O)n and CO2. Reproducible results are obtained for the number of water molecules evaporating. Nanocalorimetric analysis reveals a non-ergodic component of DelatE(ne) = 59 +/- 14 kJ mol(-1) in the core exchange reaction, most likely carried away by the neutral CO2 product. Extrapolation to solution phase values suggests hydration enthalpies of DeltaH(hyd) = -375 +/- 30 kJ mol(-1) for O2(-) and DeltaH(hyd) = -268 +/- 27 kJ mol(-1) for CO2(-).
The hydrated electron is one of the most fundamental nucleophiles in aqueous solution, yet it is a transient species in liquid water, making it challenging to study. The solvation thermodynamics of the electron are important for determining the band structure and properties of water and aqueous solutions. However, a wide range of values for the electron solvation enthalpy (-1.0 to -1.8 eV) has been obtained from previous methods, primarily because of the large uncertainty as to the value for the absolute proton solvation enthalpy. In the gas phase, electron interactions with water can be investigated in stable water clusters that contain an excess electron, or an electron and a solvent-separated monovalent or divalent metal ion. Here, we report the generation of stable water clusters that contain an excess electron and a solvent-separated trivalent metal ion that are formed upon electron capture by hydrated trivalent lanthanide clusters. From the number of water molecules lost upon electron capture, adiabatic recombination energies are obtained for La(H(2)O)(n)(3+) (n = 42-160). The trend in recombination energies as a function of hydration extent is consistent with a structural transition from a surface-located excess electron at smaller sizes (n <or= approximately 56) to a more fully solvated electron at larger sizes (n >or= approximately 60). The recombination enthalpies for n > 60 are extrapolated as a function of the geometrical dependence on cluster size to infinite size to obtain the bulk hydration enthalpy of the electron (-1.3 eV). This extrapolation method has the advantages that it does not require estimates of the absolute proton or hydrogen hydration enthalpies.
The [M21+2H]2+ cluster of the zwitterion betaine, M = (CH3)3NCH2CO2, formed via electrospray ionisation (ESI), has been allowed to interact with electrons with energies ranging from >0 to 50 eV in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. The types of gas-phase electron-induced dissociation (EID) reactions observed are dependent on the energy of the electrons. In the low-energy region up to 10 eV, electrons are mainly captured, forming the charge-reduced species, {[M21+2H]+*}*, in an excited state, which stabilises via the ejection of an H atom and one or more neutral betaines. In the higher energy region, above 12 eV, a Coulomb explosion of the multiply charged clusters is observed in highly asymmetric fission with singly charged fragments carrying away more than 70% of the parent mass. Neutral betaine evaporation is also observed in this energy region. In addition, a series of singly charged fragments appears which arise from C-X bond cleavage reactions, including decarboxylation and CH3 group transfer. These latter reactions may arise from access of electronic excited states of the precursor ions.
In solution, half-cell potentials are measured relative to other half-cells resulting in a ladder of thermodynamic values that is anchored to the standard hydrogen electrode (SHE), which is assigned an arbitrary value of exactly 0 V. A new method for measuring the absolute SHE potential is introduced in which reduction energies of Eu(H(2)O)(n)(3+), from n = 55 to 140, are extrapolated as a function of the geometric dependence of the cluster reduction energy to infinite size. These measurements make it possible to directly relate absolute reduction energies of these gaseous nanodrops containing Eu(3+) to the absolute reduction enthalpy of this ion in bulk solution. From this value, an absolute SHE potential of +4.11 V and a real proton solvation energy of -269.0 kcal/mol are obtained. The infrared photodissociation spectrum of Eu(H(2)O)(119-124)(3+) indicates that the structure of the surface of the nanodrops is similar to that at the bulk air-water interface and that the hydrogen bonding of interior water molecules is similar to that in aqueous solution. These results suggest that the environment of Eu(3+) in these nanodrops and the surface potential of the nandrops are comparable to those of the condensed phase. This method for obtaining absolute potentials of redox couples has the advantage that no explicit solvation model is required, which eliminates uncertainties associated with these models, making this method potentially more accurate than previous methods.
(H(+))(H(2)O)(n) ions (n = 1-72) at 50 keV energies were brought to collide with caesium atoms. The analysis of the products formed for clusters having n > 4 shows that this leads to the formation of a population of (OH(-))(H(2)O)(m) ions with a variable number m. On average, more than half of the water molecules are lost from the cluster in the process. A model can explain the experimental observations where two successive collisions occur within a time period of less than 100 ns. One-electron transfer from caesium to water leading to the loss of one hydrogen atom occurs at each stage. While the first stage is by itself exothermic, the second stage requires additional energy from collisional energy transfer.
The internal energy deposited into gas-phase Ru(NH3)(6)(H2O)(n)(3+) when reduced by thermal electrons is investigated as a function of cluster size. For n >= 40, reduction results exclusively in the loss of water molecules from the reduced precursor ion; loss of water is accompanied by the loss of a single ammonia molecule for smaller clusters. The average number of ligands lost from the reduced precursor decreases with cluster size for n <= 31, presumably because of increased binding energy of the ligands to the smaller, doubly charged clusters. For Ru(NH3)(6)(H2O)(55)(3+), which corresponds to a concentration or activity of about 1 M, reduction results in a mean loss of 18.2 water molecules, from which an average and maximum energy deposition of 7.9 and similar to 8.2 to 8.7 eV, respectively, is determined. To the extent that the dissociation is statistical, the internal energy deposited corresponds to the reduction energy of the hydrated precursor ion by a thermal electron in the gas phase. This measured value is combined with the electron affinity of water and the difference in solvation energies of the precursor and reduced cluster ions to provide an absolute value for the reduction energy for 1 M Ru(NH3)(6)(3+) by a solvated electron in bulk water of about -3.8 eV at 0 K. This route toward establishing absolute half-cell reduction potentials has the advantages that effects of counterions, solvent, and chemical form can be easily controlled and quantified, and redox reactions not readily observed in solution can be measured.
Infrared laser action spectroscopy in a Fourier-transform ion cyclotron resonance mass spectrometer is used in conjunction with ab initio calculations to investigate doubly charged, hydrated clusters of calcium formed by electrospray ionization. Six water molecules coordinate directly to the calcium dication, whereas the seventh water molecule is incorporated into a second solvation shell. Spectral features indicate the presence of multiple structures of Ca(H2O)(7)2+ in which outer-shell water molecules accept either one (single acceptor) or two (double acceptor) hydrogen bonds from inner-shell water molecules. Double-acceptor water molecules are predominantly observed in the second solvent shells of clusters containing eight or nine water molecules. Increased hydration results in spectroscopic signatures consistent with additional second-shell water molecules, particularly the appearance of inner-shell water molecules that donate two hydrogen bonds (double donor) to the second solvent shell. This is the first reported use of infrared spectroscopy to investigate shell structure of a hydrated multiply charged cation in the gas phase and illustrates the effectiveness of this method to probe the structures of hydrated ions.
In solution, half-cell potentials are measured relative to those of other half cells, thereby establishing a ladder of thermochemical values that are referenced to the standard hydrogen electrode (SHE), which is arbitrarily assigned a value of exactly 0 V. Although there has been considerable interest in, and efforts toward, establishing an absolute electrochemical half-cell potential in solution, there is no general consensus regarding the best approach to obtain this value. Here, ion-electron recombination energies resulting from electron capture by gas-phase nanodrops containing individual [M(NH3)6]3+, M = Ru, Co, Os, Cr, and Ir, and Cu2+ ions are obtained from the number of water molecules that are lost from the reduced precursors. These experimental data combined with nanodrop solvation energies estimated from Born theory and solution-phase entropies estimated from limited experimental data provide absolute reduction energies for these redox couples in bulk aqueous solution. A key advantage of this approach is that solvent effects well past two solvent shells, that are difficult to model accurately, are included in these experimental measurements. By evaluating these data relative to known solution-phase reduction potentials, an absolute value for the SHE of 4.2 +/- 0.4 V versus a free electron is obtained. Although not achieved here, the uncertainty of this method could potentially be reduced to below 0.1 V, making this an attractive method for establishing an absolute electrochemical scale that bridges solution and gas-phase redox chemistry.
The Thomson model, used for calculating thermodynamic properties of cluster ions from macroscopic properties, and variations of this model were compared to each other and to experimental data for both hydrated mono- and divalent ions. Previous models that used the Thomson equation to calculate sequential binding thermodynamic values of hydrated ions, either continuously or discretely including an ion-dipole interaction term, were compared to a discrete model that includes the excluded volume of an impurity ion. All models, given their limitations, provided reasonable agreement to data for monovalent ions. For divalent cluster ions, the continuous model, and a discrete model that includes the ion-exclusion volume provide significantly better agreement to both the binding enthalpy and the binding entropy data as compared to the model that includes an ion-dipole term. A systematic deviation in the continuous model resulted in significantly lower binding enthalpies than the discrete model for clusters with fewer than about nine and 19 water molecules for mono- and divalent ions, respectively, but this difference became negligible for larger clusters. Previous investigations of the various Thomson model implementations used parameters for bulk water at 313 K. Using parameters at 298 K has a negligible effect at small cluster sizes, but at larger sizes, the binding enthalpies are 0.2 kcal/mol higher than with the 313 K parameters. Although small, the effect is significant for ion nanocalorimetry experiments in which thermochemical information is obtained from the number of water molecules lost upon activating large clusters.
Hydrated trivalent rare earth metal ions containing yttrium and all naturally abundant lanthanide metals are formed using electrospray ionization, and the structures and reactivities of these ions containing 17-21 water molecules are probed using blackbody infrared radiative dissociation (BIRD) and infrared action spectroscopy. With the low-energy activation conditions of BIRD, there is an abrupt transition in the dissociation pathway from the exclusive loss of a single neutral water molecule to the exclusive loss of a small protonated water cluster via a charge-separation process. This transition occurs over a narrow range of cluster sizes that differs by only a few water molecules for each metal ion. The effective turnover size at which these two dissociation rates become equal depends on metal ion identity and is poorly correlated with the third ionization energies of the isolated metals but is well correlated with the hydrolysis constants of the trivalent metal ions in bulk aqueous solution. Infrared action spectra of these ions at cluster sizes near the turnover size are largely independent of the specific identity of the trivalent metal ion, suggesting that any differences in the structures of the ions present in our experiment are subtle.
Singly and doubly charged chromium-water ion-molecule complexes are produced by laser vaporization in a pulsed-nozzle cluster source. These species are detected and mass-selected in a specially designed time-of-flight mass spectrometer. Vibrational spectroscopy is measured for these complexes in the O-H stretching region using infrared photodissociation spectroscopy and the method of rare gas atom predissociation. Infrared excitation is not able to break the ion-water bonds in these systems, but it leads to elimination of argon, providing an efficient mechanism for detecting the spectrum. The O-H stretches for both singly and doubly charged complexes are shifted to frequencies lower than those for the free water molecule, and the intensity of the symmetric stretch band is strongly enhanced relative to the asymmetric stretch. Partially resolved rotational structure for both complexes shows that the H-O-H bond angle is greater than it is in the free water molecule. These polarization-induced effects are enhanced in the doubly charged ion relative to its singly charged analog.
Electron-capture dissociation (ECD) is a new fragmentation technique that utilizes ion-electron recombination reactions. The latter have parallels in other research fields; revealing these parallels helps to understand the ECD mechanism. An overview is given of ECD-related phenomena and of the history of ECD discovery and development. Current views on the ECD mechanism are discussed using both published and new examples.
Several collaborative efforts involving the Coe group are described that connect the properties of aqueous clusters to bulk. Success in connecting cluster properties to bulk has led to new insights and reassessed properties of bulk water including, firstly, the determination of the absolute bulk hydration enthalpy and free energy of the proton using experimental clustering data on A ± (H2O)n ions and, secondly, the determination of the bandgap V0 and a new energy diagram for bulk water using experimental detachment and dissociation spectra of (H2O)n- hydrated electron species. The results should be of interest to anyone studying ions in water.
Various thermodynamic equilibrium properties of naturally abundant, hexagonal ice ice Ih of water (H 2 O) have been used to develop a Gibbs energy function g(T, p) of temperature and pressure, covering the ranges 0–273.16 K and 0 Pa–210 MPa, expressed in the temperature scale ITS-90. It serves as a fundamental equation from which addi-tional properties are obtained as partial derivatives by thermodynamic rules. Extending previously developed Gibbs functions, it covers the entire existence region of ice Ih in the T-p diagram. Close to zero temperature, it obeys the theoretical cubic limiting law of Debye for heat capacity and Pauling's residual entropy. It is based on a significantly enlarged experimental data set compared to its predecessors. Due to the inherent thermo-dynamic cross relations, the formulas for particular quantities like density, thermal ex-pansion, or compressibility are thus fully consistent with each other, are more reliable now, and extended in their ranges of validity. In conjunction with the IAPWS-95 formu-lation for the fluid phases of water, the new chemical potential of ice allows an alternative computation of the melting and sublimation curves, being improved especially near the triple point, and valid down to 130 K sublimation temperature. It provides an absolute entropy reference value for liquid water at the triple point. © 2006 American Institute of Physics.
This book provides a penetrating and comprehensive description of energy selected reactions from a theoretical as well as experimental view. Three major aspects of unimolecular reactions involving the preparation of the reactants in selected energy states, the rate of dissociation of the activated molecule, and the partitioning of the excess energy among the final products, are fully discussed with the aid of 175 illustrations and over 1,000 references, most from the recent literature. Examples of both neutral and ionic reactions are presented. Many of the difficult topics are discussed at several levels of sophistication to allow access by novices as well as experts. Among the topics covered for the first time in monograph form is a discussion of highly excited vibrational/rotational states and intramolecular vibrational energy redistribution. Problems associated with the application of RRKM theory are discussed with the aid of experimental examples. Detailed comparisons are also made between different statistical models of unimolecular decomposition. Both quantum and classical models not based on statistical assumptions are described. Finally, a chapter devoted to the theory of product energy distribution includes the application of phase space theory to the dissociation of small and large clusters. The work will be welcomed as a valuable resource by practicing researchers and graduate students in physical chemistry, and those involved in the study of chemical reaction dynamics.
The internal energy distributions P(ε) transferred to W(CO)
6+· during the kiloelectronvolt collisions that occur upon neutralization-reionization (NR) have been estimated based on the relative abundances of the W(CO)
0−6+· products present in NR spectra (thermochemical method). The average internal energy of the incipient {W(CO)
6+·}* ions arising after near thermoneutral neutralization with trimethylamine followed by reionization with O2 is ∼9 eV for 8-keV precursor ions and is mainly deposited during reionization, For comparison, the mean internal energy of {W(CO)
6+·}* after electron ionization (EO or collisionally activated dissociation (CAD) is ∼ 6 eV. Making the neutralization step endothermic slightly increases the overall excitation gained; however, a large increase in endothermicity (> 16 eV) causes only a modest rise of the average internal energy (<2 eV). The P(ε) curve for NR increases exponentially up to ∼ 6 eV and levels off at higher energies.. showing that the probability of imparting large internal energies (6–17 eV) is high. In sharp contrast, the most probable excitation on CAD is ≤6 eV, and the probability of deposition of larger energies declines exponentially. The mean internal energies after CAD and NR decrease steadily when the kinetic energy is lowered. The structure (minima-maxima) observed in the P(ε) distribution for El, which most likely originates from Franck-Condon factors, is not reproduced in the distributions for NR or high energy CAD, despite the fact that all three methods involve electronic excitation. Because of the large internal energies transferred upon NR, NR mass spectrometry could be particularly useful in the differentiation of ionic isomers with high dissociation but low isomerization thresholds.
A new apparatus for making blackbody infrared radiative dissociation (BIRD) measurements at below ambient temperature is described, and its use for measuring threshold dissociation energies of weakly bound clusters is demonstrated. Hydration energies are determined for alkaline-earth metal water clusters, X2+(H2O)n, X=Mg, Ca and n=8-10. For n=8 and 9, the energies obtained from BIRD measurements are in excellent agreement with values reported previously, but for n=10, the energies are slightly lower than those determined previously using the high-pressure ion source mass spectrometry (HPMS) equilibrium method.
For Abstract see ChemInform Abstract in Full Text.
Thresholds for collision-induced dissociation of M+(H2O)x (x = 1-4, M = Na, Mg, and Al) with xenon are measured by using guided ion beam mass spectrometry. In all cases, the primary product is endothermic loss of one water molecule. The cross-section thresholds are interpreted to yield 0 K bond energies after accounting for the effects of multiple ion-molecule collisions, internal energy of the clusters, and dissociation lifetimes. Our results are in good agreement with previous experimental results for Na+(H2O)x and in reasonable agreement with theoretical predictions for all systems. Trends in bond dissociation energies with increasing ligation and for the various metals studied are discussed.
Photodissociation spectra of Mg+(H2O)n (n=1–5) cluster ions were examined in the wavelength region from 720 to 250 nm by monitoring the total yield of the fragment ions. The absorption bands exhibit redshifts as large as 17 000 cm−1 with respect to the 2P–2S resonance line of the free Mg+ ion and were explained by the shift of this transition as a result of hydration. The spectra also exhibit clear evolution of solvation shell with the first shell closing at n=3, being consistent with the theoretical prediction. The mass spectra of the fragment ions show the existence of two dissociation processes: the evaporation of water molecules and the photoinduced intracluster reaction to produce the hydrated MgOH+ ion, MgOH+(H2O)m. The branching fraction between the two processes depends strongly on the solvent number n and also on the photolysis wavelength. The energetics and the dynamics of the dissociation processes were discussed in conjunction with the results of abinitio calculations.
Unimolecular fragmentation and bimolecular reactions with HCl of water clusters which nominally contain Mg+ cations were studied in an FT-ICR spectrometer. A cluster fragmentation and successive evaporation of single water molecules occurring on a millisecond timescale and driven by ambient black body radiation is triggering interesting intracluster reactions. Below a certain critical size (∼17 water molecules) MgOH+ forms, and a hydrogen atom is ejected. Similarly bimolecular reactions of Mgaq+ clusters with HCl result in a release of H atom and formation of MgClaq+. Both findings can be rationalized by assuming that the solvated Mg+ cations actually detach an additional electron forming a Mgaq2+ and eaq− within clusters with more than 17 water molecules. Mg+ formed by recombination when not enough solvent is available to stabilize the separate charged species then reacts with a water molecule resulting in H-atom formation. Detailed studies of the ion reactions and fragmentation provide additional insights into the structure and stability of solvated magnesium cations.
Structures of hydrated singly positive charged calcium−water clusters Ca+(H2O)n and their hydrogen-eliminated products (CaOH)+(H2O)n-1 were optimized using the ab initio molecular orbital methods and are compared with cationic magnesium−water clusters which have been investigated previously. For n ≥ 2, the structures of Ca+(H2O)n are different from those of Mg+(H2O)n. In the Mg+(H2O)n clusters, a pyramidical Mg+(H2O)3 forms the first shell. In contrast, a quasi-square-planar Ca+(H2O)4 is the first shell. The structures of (CaOH)+(H2O)n-1 are also different from structures (MgOH)+(H2O)n-1. The structural difference is attributed to the participation of the d orbitals of Ca atom in the bonding. Despite these structural differences, the core molecular ion (CaOH)+ in the hydrogen-eliminated products (CaOH)+(H2O)n-1 is very similar to the corresponding core ion (MgOH)+. Both ions, CaOH+ and MgOH+, are strongly polarized to Ca2+O-H and Mg2+O-H. Consequently, the hydration energies of the (CaOH)+(H2O)n are much larger than those of the corresponding Ca+(H2O)n. The internal energy change of the hydrogen-elimination reactions of the Ca+(H2O)n is positive for n = 1−4 but becomes negative for n ≥ 5, which is consistent with the product switch in the time-of-flight mass spectrum reported by Fuke's group. The equilibrium constants of the hydrogen elimination reaction are also consistent with the experimental observed isotope effects and the determined metal dependencies.
Infrared photodissociation spectra of [Mg·(H2O)1-4]+ and [Mg·(H2O)1-4·Ar]+ are measured in the 3000−3800 cm-1 region. For [Mg·(H2O)1-4]+, cluster geometries are optimized and vibrational frequencies are evaluated by density functional theory calculation. We determine cluster structures of [Mg·(H2O)1-4]+ by comparison of the infrared photodissociation spectra with infrared spectra calculated for optimized structures of [Mg·(H2O)1-4]+. In the [Mg·(H2O)1-3]+ ions, all the water molecules are directly bonded to the Mg+ ion. The infrared photodissociation spectra of [Mg·(H2O)4]+ and [Mg·(H2O)4·Ar]+ show bands due to hydrogen-bonded OH stretching vibrations in the 3000−3450 cm-1 region. In the [Mg·(H2O)4]+ ion, three water molecules are attached to the Mg+ ion, forming the first solvation shell; the fourth molecule is bonded to the first solvation shell. As a result, the most stable isomer of [Mg·(H2O)4]+ has a six-membered ring composed of the Mg+ ion, two of the three water molecules in the first solvation shell, and a termination water molecule.
Ab initio electronic structure calculations on model cations containing a disulfide linkage and a protonated amine site are carried out to examine how the rate of electron transfer from a Rydberg orbital on the amine site to the SS σ* orbital depends upon the distance between these two orbitals. These simulations are relevant to both electron-capture and electron-transfer dissociation mass spectrometry where protonated peptide or protein samples are assumed to capture electrons in Rydberg orbitals of their protonated sites subsequent to which other bonds (especially SS and NCα) are cleaved. By examining the dependence of three diabatic potential energy surfaces (one with an electron in the ground-state Rydberg orbital of the protonated amine, one with the electron in an excited Rydberg orbital on this same site, and the third with the electron attached to the SS σ* orbital) on the SS bond length, critical geometries are identified at which resonant through-bond electron transfer (from either of the Rydberg sites to the SS σ* orbital) can occur. Landau–Zener theory is used to estimate these electron transfer rates for three model compounds that differ in the distance between the protonated amine and SS bond sites. Once the electron reaches the SS σ* orbital, cleavage of the SS bond occurs, so it is important to characterize these electron transfer rates because they may be rate-limiting in at least some peptide or protein fragmentations. It is found that the Hamiltonian coupling matrix elements connecting each of the two Rydberg-attached states to the σ*-attached state decay exponentially with the distance between the Rydberg and σ* orbitals, so it is now possible to estimate the electron transfer rates for other similar systems.
Thermochemical aspects of the cooling of isolated liquid drops by evaporation are considered, with explicit reference to amorphous water. Monte Carlo simulations are used to examine the role of fluctuations in energy dissipation and the effect of local stabilities on the terminal cluster size. Kinetic aspects of the cooling are then considered. We establish the pertinence of our studies to clusters generated in sonic nozzle expansions, and show that they will be metastable. We then find that a given mass‐selected cluster will comprise a broad range of internal energies. Severe constraints on the kinetic modeling of evaporation patterns are thus imposed. These are illustrated by reference to data in the literature.
The vertical and adiabatic ionization energies as well as the spatial volumes of the singly occupied molecular orbital (SOMO) of [Mg,nH2O]+, n ⩽ 19, were determined by ab initio calculations. Ionization energies were evaluated from Koopmans’ theorem and explicitly as differences of the total energies of [Mg,nH2O]+ and [Mg,nH2O]2+ as obtained by Hartree–Fock, post-Hartree–Fock and gradient corrected density functional (DFT) methods. In the case of clusters with a sixfold coordinated magnesium cation [Mg(H2O)6,(n−6)H2O]+ Koopmans’ theorem fails for n = 6–8,10. In contrast this is a valid approximation for all other cluster sizes. The most stable isomers of [Mg,nH2O]+, n = 6–9, exhibit significantly enhanced SOMO volumes. This coincides with a significant drop in ionization energies and with an increase in electron correlation. In these clusters Koopmans’ theorem is a crude approximation due to the neglect of electron correlation. The cluster size dependency of orbital relaxation and change in electron correlation upon ionization allows for an analytical fit in terms of the spatial SOMO volume. Reorganization energies and SOMO volumes indicate strong structural changes in the clusters during ionization due to a significant localization of the SOMO in [Mg,nH2O]+, n<6 and n>8. © 2003 American Institute of Physics.
The binding energies of water cluster cations are obtained by measuring decay fractions of metastable dissociation and employing Klots’ model of evaporative dissociation. Their variation with degree of solvation shows the commonly observed decrease, followed by a slow rise in magnitude, which typifies the trend found for solvated cations. There is no observed abrupt change in the vicinity of the well‐known magic number (H2O)21⋅H+ corresponding to (H2O)20⋅H3O+. Other data are used to deduce free energies for water clusters up to size n=28, allowing a determination of entropy changes with size. All of the thermochemical data, including prior literature values, are assessed in terms of calculations made using the liquid drop model and standard statistical mechanical equations. It is concluded that entropic rather than energetic effects give rise to the referred to magic number.
Experimental observations of the effects of solvation on the dehydrogenation reaction of Mg+(H2O)n to produce MgOH+(H2O)n−1 are presented for n≤6. The reaction is seen to occur spontaneously at room temperature for n≳4. Ligand switching reactions are used to show the Mg+–OH bonds are stronger than Mg+H2O bonds. The results show the energy required to lose an H atom decreases with the number of water molecules attached because the magnesium changes oxidation state and this results in stronger interactions with the water ligands. Ab initio calculations are used to explain these observations.
The average increases in the internal energy of ions when excited by photons and in collisions with gas molecules have been compared. The positive molecular ions of n-butylbenzene and n-pentylbenzene were used with collisional excitation taking place in the second field-free region of a ZAB-2F mass spectrometer. The ratios of the intensifies of the peaks of the fragment ions C7H7+ and C7H8+ were measured and compared with similar data from photodissociation experiments in which the excitation energy is known to high accuracy. This procedure gives the average collisional excitation energies, i.e. the single-valued excitation energies which would be needed to produce the same dissociation pattern as is produced by the collision gas. These were investigated as a function of the nature of the collision gas, the gas pressure and translational energy of the ions entering the collision region.
The 1-hydroxy-1-(N-methyl)aminoethyl radical (1) represents a simple model system for hydrogen atom adducts to the amide bond in gas-phase peptide and protein ions relevant to electron capture dissociation (ECD). Radical 1 was generated in the rarefied gas phase by femtosecond electron transfer to the stable cation prepared by selective O-protonation of N-methylacetamide. The main dissociations of 1 were loss of the hydroxyl hydrogen atom and the N-methyl group in a 1.7:1 ratio, as deduced from product analysis and deuterium labeling. The dissociations that occur on the 4.1 microsecond time scale are driven by large Franck−Condon effects on collisional electron transfer that deposit 93−103 kJ mol-1 in the nascent radicals. Detailed analysis of the potential energy surface for dissociations of 1 revealed several conformers and isomeric transition states for dissociations of the O−H and N−CH3 bonds. The experimental branching ratio is in quantitative agreement with RRKM calculations within the accuracy of the G2 potential energy surface and favors cleavage of the O—H bond in 1 and loss of H. This finding contrasts previously reported results, as discussed in the paper.
The amino(hydroxy)methyl radical (1) represents the simplest model for hydrogen atom adducts to the amide bond. Radical 1 was generated in the gas phase by femtosecond electron transfer to protonated formamide and found to be stable on the microsecond time scale. The major unimolecular dissociation of 1 was loss of the hydrogen atom from the hydroxyl group. Losses of hydrogen atoms from the CH and NH2 groups in 1 were less abundant. RRKM calculations on the G2(MP2), G2, and CCSD(T)/aug-cc-pVTZ potential energy surfaces predicted preferential loss of the hydroxyl hydrogen atom, in qualitative agreement with experiment. Bimolecular reactions of hydrogen atoms with formamide were predicted by calculations to prefer H atom abstraction from the H−C bond forming H2 and NH2CO•. This reaction was calculated to be 43 kJ/mol exothermic and had to overcome an activation energy barrier of 28.5 kJ/mol. Hydrogen atom additions to the carbon and oxygen termini of the carbonyl group in formamide had similar activation energies, 51 and 49 kJ/mol, respectively. H atom addition to the C-terminus producing the aminomethyloxy radical (6) was calculated to be 2 kJ/mol endothermic.
The hydrates H+(H2O)n were observed in irradiated water vapor at pressures from 0.1 to 6 torr and temperatures from 15 to 600°. Extermination of the relative concentrations of the hydrates after mass analysis of the ions allows determination of the equilibrium constants Kn-1,n for the reactions H+(H2O)n-1 + H2O ⇄ H+-(H2O)n. Determination of the equilibrium constants over the experimental range leads to values for ΔHn-1,n, ΔG°n-1,n, and ΔS°n-1,n. The experimental values for -ΔHn-1,n, and -ΔG°n-1,n in brackets, are: (1,2) 36 [25]; (2,3) 22.3 [13.6]; (3,4) 17 [8.5]; (4,5) 15.3 [5.5]; (5,6) 13 [3.9]; (6,7) 11.7 [2.8]; (7,8) 10.3 [2.2] kcal/mole. The free-energy data are for 300°K and standard state of 1 atm. From these data one can calculate the hydrate distribution over a wide range of pressures and temperatures. It is shown that the ΔH and ΔS values are of reasonable magnitude. The continuous decrease of the ΔHn-1,n and ΔGn-1,n values shows that the stabilities of the hydrates H+(H2O)n change quite continuously. No single structure shows dominant stability. Comparison of the data with the proton affinity of water suggests that in the lower hydrates (n = 2 to 4 or even 6) all water molecules are equivalent. This would make the notation H3O+(H2O)n inappropriate. The data also indicate that beyond n = 4 either a new "shell" is started or crowding of the first "shell" occurs.
With ab initio molecular orbital calculations, the structures of the cation clusters Mg+(H2O)(n) and their hydrogen-eliminated products (MgOH)(+)(H2O)(n-1) are optimized. In Mg+(H2O)(n), the hydration number of the most stable isomer is 3. In (MgOH)(+)(H2O)(n-1), all water molecules are directly bonded to Mg+ for n less than or equal to 6. The hydration energy of (MgOH)(+) is larger than that of Mg+ because of the strongly polarized (MgOH)(+) molecular ion; Mg is oxidized halfway to Mg(II). The internal energy change of the hydrogen elimination of Mg+(H2O)(n) is positive for n = 1-5, but becomes negative for n = 6, which is in good agreement with the product switching in the TOF spectrum reported in the preceding paper by Sanekata et al. The effects of isotope substitution and equilibrium constants of the hydrogen (deuterium) elimination reaction observed in their experiment can be explained qualitatively.
The reactions of Mg+ and Ca+ ions with water clusters are examined using a reflectron time-of-flight mass spectrometer combined with a laser vaporization technique. Both the M(+)(H2O)(n) and MOH(+)(H2O)(n-1) (M = Mg and Ca) ions are found to form as the reaction products with characteristic size distributions: the latter ions are produced via an H-atom elimination reaction (oxidation of M(+)). As for the Mg+ ion, the Mg+(H2O)(n) ions are predominantly produced for 1 less than or equal to n less than or equal to 5 and n greater than or equal to 15, while MgOH+(H2O)(n-1) are exclusively observed for 6 less than or equal to n less than or equal to 14 in the mass spectrum. Similar product distributions are also observed for Mg+-D2O, Ca+-H2O, and Ca+-D2O systems, though they are found to be affected by deuterium and metal substitutions. On the basis of these results as well as those on the photoinduced reaction of Mg+(H2O)(n) reported previously, the first product switching at n = 5 for Mg+ (n = 4 for Ca+) is ascribed to the difference in the successive hydration energies of the M(+) and MOH(+) ions. As for the second product switching, two possible mechanisms are proposed such as the stabilization of a Rydberg-type ion-pair state and the involvement of a new product.
The photochemistry of n-butylbenzene ions was studied in the ion cyclotron resonance ion trap using Fourier-transform and double-resonance techniques. The branching ratio for formation of the m/e 92 and 91 product ions was determined at several wavelengths. The effect of thermal excitation of the ions was analyzed, and values were derived for the branching ratio as a function of ion internal energy. The results are in agreement with dissociative charge-transfer ionization experiments, after similar correction for thermal excitation, but previous ion-beam photodissociation results are badly divergent as a result of large excess internal energy of the parent ions. RRKM calculations of the branching ratio suggest that the dissociation behavior is consistent with a quasiequilibrium-theory description, and that the activation energy for m/e 91 formation is 0.6 eV higher than that for m/e 91 formation. The actual values of these activation energies are less well determined, but the results are consistent with a value of 1.1 eV for m/e 92 formation, in agreement with existing thermochemical information, assuming a methylenecyclohexadiene product ion structure.
The dissociation dynamics of n-butylbenzene ions have been measured as a function of the parent ion internal energy. Ions were produced by dispersed synchrotron radiation and energy selected by photoelectron photoion coincidence. The branching ratio of the two fragment ions of mass 91 and 92 was measured over an ion internal energy range from 2 to 6.5 eV. This ratio, which varies from 0.05 to 8.0 is a useful thermometer for the n-butylbenzene ion internal energy. The measured dissociation rates for the production of C7H8+ were modeled with RRKM/QET calculations, from which an activation energy of 0.99 eV was derived. The assumption of a tight transition state with ΔS‡ = -7 cal/K is necessary in order to account for the slow rise in the dissociation rate with ion internal energy. The small kinetic energy released in the dissociation indicates that the rate-determining step is an isomerization reaction, prior to the actual dissociation.
Electronically excited states of magnesium-water cluster ions, Mg+(H2O)n, are studied by photodissociation after mass selection. Dissociation spectra are obtained as a function of wavelength for n = 1 (250-370 nm) and 2 (280-470 nm). The spectra show absorption peaks at 28 300, 30 500, and 38 500 cm-1 for n = 1, and 25 000, 29 400, and 32 000 cm-1 for n = 2. These absorption bands are assigned to the 2P-2S type transitions localized on the Mg+ ion with the aid of ab initio CI calculations. In addition to evaporation of water molecules, photoinduced chemical reaction to produce MgOH+ is found to occur efficiently. Especially, for Mg+(H2O)2, the branching ratio between the former and latter processes has been found to depend sensitively on the excitation energy. On the basis of these results, the mechanism of the photodissociation of these cluster ions is discussed.
A technique has been developed for electron impact excitation of ions from organics (EIEIO). The technique uses trapped ion cyclotron resonance (ICR) spectroscopy and together with the ICR photochemical techniques and unusual capabilities for studying ion-molecule reactions, will aid in the evolution of ICR as an analytical instrument. It Is demonstrated on a variety of substituted benzene radical cations. Ions are generated and subsequently excited in a continuous electron beam while being trapped in the source region. The spectra obtained by EIEIO are shown to be analogous to those obtained by cofflsion induced dissociation and yield characteristic structural information.
Energy and angular distributions of H-(D-) ions formed by dissociative attachment in H2O(D2O) have been observed for the processes situated in the region of 6.5, 8.6 and 11.8 eV. The angular behaviour would indicate that the symmetries of the resonant states responsible for these processes are 2B1, 2A1 and 2B2, respectively. The energy distributions of the H-(D-) ions for all three processes show that most of the dissociation energy is in the form of translation and furthermore the distributions for each process and isotope are similar. This similarity would indicate that the potential surfaces and the autodetachment widths of the H2O- states are also about the same which would support the proposals and calculations of Jungen, Vogt and Staemmler (1979) who predict that these three states are formed by two electrons in 3sa1 orbitals bound to an H2O+ core with a vacancy in the 1b1, 3a1 and 1b2 orbitals, respectively.
Ab initio electronic structure calculations are used to explore the effect of nonneighboring positively charged groups on the ability of low-energy (<1 eV) electrons to directly attach to S-S σ bonds in disulfides to effect bond cleavage. It is shown that, although direct vertical attachment to the σ* orbital of an S-S σ bond is endothermic, the stabilizing Coulomb potential produced in the region of the S-S bond by one or more distant positive groups can render the S-S σ* anion state electronically stable. This stabilization, in turn, can make near vertical electron attachment exothermic. The focus of these model studies is to elucidate a proposed mechanism for bond rupture that may, in addition to other mechanisms, be operative in electron capture dissociation (ECD) experiments. The importance of these findings lies in the fact that a more complete understanding of how ECD takes place will allow workers to better interpret ECD fragmentation patterns observed in mass spectrometric studies of proteins and polypeptides.
Collision of the title ion upon a stainless steel surface at near-normal incidence leads to deposition of internal energy in a well-defined narrow distribution. The energy deposited increases with laboratory collision energy and exceeds 7 eV (average) for 100 eV collisions. The translational-to-vibrational energy transfer efficiency is 15% (assuming an infinitely massive target) at 25 eV collision energy. Comparison is made with the internal energy distributions associated with gas-phase collisional activation using both low and high ion kinetic energies. The narrowness of the distribution of internal energies, the easy access to ions excited to different extents, and the high internal energies accessible, make the ion/surface collision process superior to gas-phase collisional activation for this system.
Electron capture and loss cross-sections have been measured for collisions between fast multiply charged peptide and protein
ions [M+
nH]n+ and Na or C60. The ions were produced in an electrospray ion source (ESI) and accelerated to an energy of n
×50 keV. We find that the size of the cross-sections depend strongly on molecule size, ionization energy and projectile charge
state n. For multiply charged ubiquitin projectiles, the cross-sections for electron capture are found to be several times larger
with a Na target than with a C60 target. This observation is qualitatively explained by means of an over-the-barrier model for electron transfer using metal-sphere
descriptions for the electronic responses of the collision partners.
The mode of electron interaction (attachment) changes drastically when going from an isolated molecule to a cluster consisting of these molecules. It is the purpose of this review to focus on electrons in clusters and in particular to summarize differences between a negatively charged isolated molecule and its aggregated counterpart. First the physical properties of negative cluster ions (electron affinity, electronic states) and then kinetics of free electron attachment (attachment cross-sections) will be presented for the van der Waals clusters studied so far.
The identity of neighboring amino acids has little influence on the dissociation of multiply protonated proteins by electron capture dissociation. As exceptions, no cleavage occurs on the N-terminal side of Pro, and little on either side of Cys, whereas the C-terminal side of Trp is heavily favored. The neighboring amino acids have a far greater effect on energetic dissociation, making the combined methods promising for the de novo sequencing of proteins.
High-performance liquid chromatography (HPLC) has been combined with high-resolution ion mobility separations and time-of-flight mass spectrometry (MS) for the analysis of complex biomolecular mixtures such as those that arise upon tryptic digestion of protein mixtures. In this approach, components in a mixture are separated using reversed phase HPLC. As mixtures of peptides exit the column, they are electrosprayed into an ion mobility/time-of-flight mass spectrometer. Mixtures of ions are separated based on differences in mobilities through a buffer gas, and subsequently dispersed by differences in mass-to-charge (m/z) ratios in a mass spectrometer. The multidimensional approach is feasible because of the large differences in timescales of the HPLC (minutes), ion mobility (milliseconds), and time-of-flight (microseconds) techniques. Peak capacities for the two-dimensional liquid chromatography-ion mobility separations (LC-IMS) and three-dimensional LC-IMS-MS separations are estimated to be ∼900 to 1 200 and ∼3.7 to 4.6 × 105, respectively. The experimental apparatus and data acquisition considerations are described; data for a mixture of peptides obtained upon tryptic digestion of five proteins (albumin, bovine and pig; cytochrome c, horse; hemoglobin, dog and pig) are presented to illustrate the approach.
Beside the known maximum of electron capture dissociation (ECD) of gas-phase polypeptide polycations at zero electron energy, a broad local maximum is found around 10 eV. This maximum is due to electronic excitation prior to electron capture, as in dissociative recombination of small cations. In the novel hot electron capture dissociation (HECD) regime, not only N–Cα bonds are cleaved as in ECD, but secondary fragmentation is also induced due to the excess energy. Beneficially, this fragmentation includes abundant losses of from leucine and from isoleucine residues terminal to the cleavage site, which allows for distinguishing between these two isomeric residues.
It is possible to gain information about internal energy distributions of ions knowing their mass spectra and the energetics of fragmentation if one can equate pre-exponential terms in the unimolecular rate expressions. Positively charged triethyl phosphate, which fragments via a linear sequence of unimolecular reactions, has been examined in this way and the results have been checked by back-calculating the mass spectrum. It is found that low energy (28 eV in laboratory frame, argon target) collision-induced dissociation deposits a comparable average internal energy to what occurs at high energy (7 keV). High energy collisions, however, result in a distribution of internal energies which includes a low probability tail representing very highly excited ions. In the low energy regime, it is possible to increase dramatically the effective internal energy deposited by raising the collision gas pressure. The method of estimating internal energy distributions given here is particularly attractive for comparing ions excited by various means.
Dissociation induced by electron capture in high energy collisions between doubly protonated peptide ions and Na atoms has been investigated. The ions were produced in an electrospray ion source and accelerated to an energy of 100 keV before they were excited in collisions with a Na target gas. Electron capture was found to be the dominant reaction channel but also fragment peaks corresponding to cleavage of the backbone N-C-alpha bonds the so called c and z ions are prominent in the recorded mass spectra, Electron capture dissociation (ECD) where free electrons a-re captured by ions stored in a FTICR cell has previously been shown to result in sequence information. Similar measurements have been performed in both a Ne and a Mg target and by comparing the mass spectra for the three target gases it is concluded that electron capture by protonated peptides in high energy collisions leads to non-ergodic fragmentation of the peptide ion. (C) 2002 Elsevier Science B.V. All rights reserved.
Extensively hydrated divalent metal ions are used as nanocalorimeters to measure the internal energy deposition resulting from electron capture. For M(H2O)(32)(2+), M = Mg, Ca, Sr, and Ba, two dissociation pathways are observed: loss of a water molecule from the precursor (similar to 6%) owing to activation from blackbody photons or collisions with residual background gas, and loss of between 9 and 11 water molecules from the reduced precursor formed by electron capture. The binding energy of a water molecule to the reduced precursor ions is estimated to be approximately 10 kcal/mol. From the distribution of water molecules lost, corrected for residual activation, the average and maximum internal energy deposited into these ions is determined to be similar to 4.4 and similar to 4.8-5.2 eV, respectively. The average internal energy deposition does not depend significantly on metal ion size (similar to 0.1 eV difference for Mg to Ba) despite the large (5.0 eV) difference in second ionization energies for the isolated atoms. Similar results were obtained for Ca(H2O)(n)(2+), n = 30 and 32, suggesting that neither the water binding energy nor the recombination energy changes significantly for clusters of this size. The recombination energy is roughly estimated from theory to be about 4.5 eV. These results show that these ions are not significantly activated by inelastic non-capture collisions with electrons and that the vast majority of the recombination energy resulting from electron capture is converted into internal energy of the reduced precursor ions, indicating that these ions dissociate statistically.
Dissociative electron attachment (DEA) to water in the gaseous phase has been studied using two different crossed electron–molecule beam apparatus. Ion yields for the formation of the three fragments H-, O- and OH- were measured as a function of the incident electron energy. The kinetic energies of the fragment ions were measured and compared with the values derived from ab initio calculations to provide information on the energy partitioning in the fragmentation process. Isotope and temperature effects on the attachment process are discussed and the production of OH- via DEA is confirmed.
A charged particle in a uniform moving magnetic field H describes a circular orbit in a plance perpendicular to H with an angular frequency or "cyclotron frequency" omagae. When an alternating electric field E(t) is applied normal to H at omegae, the ions absorb energy from the alternating electric field, and are accelerated to larger velocities and orbital radii. [1] The absorption of energy from E(t) at the cyclotron resonance frequency can be conveniently detected using a marginal oscillator detector. When the ions accelerated by E(t) collide with other particles, they lose some of their excess energy. A mixture of ions and neutral molecules in the presence of H and E(t) then reaches a steady-state condition in which the energy gained by the ions from E(t) between collisions is lost to the neutral molecules in collisions.
Forming hydrated clusters containing triply charged metal ions is challenging due to the competing process of dissociation by forming the metal hydroxide with one less net charge and a protonated water molecule. It is demonstrated for the first time that it is possible to form such clusters using a method we call "nanodrop mass spectrometry". Clusters of the form [M(H(2)O)(n)](3+), where M = Ce, Eu, and La, are generated using electrospray ionization and are mass analyzed in a Fourier-transform ion cyclotron resonance mass spectrometer with an ion cell cooled to -140 °C. Clusters containing trivalent La with n ranging from 16 to over 160 can be readily produced. These clusters are stable at this temperature for many seconds, enabling all standard methods to probe structure and reactivity of these unusual species. Photodissociation experiments on extensively hydrated clusters of trivalent lanthanum using resonant infrared radiation indicate that a minimum of 17 water molecules is necessary to stabilize these trivalent clusters under the low-energy ion excitation conditions and long time frame of these experiments. These results indicate that a minimum droplet size of approximately a nanometer is necessary for these trivalent species to survive intact. This suggests that elemental speciation of trivalent metal ions from aqueous solutions should be possible using nanodrop mass spectrometry.
Mild gas-phase acids C4H9+ and NH4+ protonate pyrrole at C-2 and C-3 but not at the nitrogen atom, as determined by deuterium labeling and neutralization-reionization mass spectrometry. Proton affinities in pyrrole are calculated by MP2/6-311G(2d,p) as 866, 845 and 786 kJ mol-1 for protonation at C-2, C-3 and N, respectively. Vertical neutralization of protonated pyrrole generates bound radicals that in part dissociate by loss of hydrogen atoms. Unimolecular loss of hydrogen atom from C-2- and C-3-protonated pyrrole cations is preceded by proton migration in the ring. Protonation of gaseous imidazole is predicted to occur exclusively at the N-3 imine nitrogen to yield a stable aromatic cation. Proton affinities in imidazole are calculated as 941, 804, 791, 791 and 724 for the N-3, C-4, C-2, C-5 and N-1 positions, respectively. Radicals derived from protonated imidazole are only weakly bound. Vertical neutralization of N-3-protonated imidazole is accompanied by large Franck-Condon effects which deposit on average 183 kJ mol-1 vibrational energy in the radicals formed. The radicals dissociate unimolecularly by loss of hydrogen atom, which involves both direct N-H bond cleavage and isomerization to the more stable C-2 H-isomer. Potential energy barriers to isomerizations and dissociations in protonated pyrrole and imidazole isomers and their radicals were investigated by ab initio calculations.
For proteins of < 20 kDa, this new radical site dissociation method cleaves different and many more backbone bonds than the conventional MS/MS methods (e.g., collisionally activated dissociation, CAD) that add energy directly to the even-electron ions. A minimum kinetic energy difference between the electron and ion maximizes capture; a 1 eV difference reduces capture by 10(3). Thus, in an FTMS ion cell with added electron trapping electrodes, capture appears to be achieved best at the boundary between the potential wells that trap the electrons and ions, now providing 80 +/- 15% precursor ion conversion efficiency. Capture cross section is dependent on the ionic charge squared (z2), minimizing the secondary dissociation of lower charge fragment ions. Electron capture is postulated to occur initially at a protonated site to release an energetic (approximately 6 eV) H. atom that is captured at a high-affinity site such as -S-S- or backbone amide to cause nonergodic (before energy randomization) dissociation. Cleavages between every pair of amino acids in mellitin (2.8 kDa) and ubiquitin (8.6 kDa) are represented in their ECD and CAD spectra, providing complete data for their de novo sequencing. Because posttranslational modifications such as carboxylation, glycosylation, and sulfation are less easily lost in ECD than in CAD, ECD assignments of their sequence positions are far more specific.
The structural characterization of proteins expressed from the genome is a major problem in proteomics. The solution to this problem requires the separation of the protein of interest from a complex mixture, the identification of its DNA-predicted sequence, and the characterization of sequencing errors and posttranslational modifications. For this, the "top down" mass spectrometry (MS) approach, extended by the greatly increased protein fragmentation from electron capture dissociation (ECD), has been applied to characterize proteins involved in the biosynthesis of thiamin, Coenzyme A, and the hydroxylation of proline residues in proteins. With Fourier transform (FT) MS, electrospray ionization (ESI) of a complex mixture from an E. coli cell extract gave 102 accurate molecular weight values (2-30 kDa), but none corresponding to the predicted masses of the four desired enzymes for thiamin biosynthesis (GoxB, ThiS, ThiG, and ThiF). MS/MS of one ion species (representing approximately 1% of the mixture) identified it with the DNA-predicted sequence of ThiS, although the predicted and measured molecular weights were different. Further purification yielded a 2-component mixture whose ECD spectrum characterized both proteins simultaneously as ThiS and ThiG, showing an additional N-terminal Met on the 8 kDa ThiS and removal of an N-terminal Met and Ser from the 27 kDa ThiG. For a second system, the molecular weight of the 45 kDa phosphopantothenoylcysteine synthetase/decarboxylase (CoaBC), an enzyme involved in Coenzyme A biosynthesis, was 131 Da lower than that of the DNA prediction; the ECD spectrum showed that this is due to the removal of the N-terminal Met. For a third system, viral prolyl 4-hydroxylase (26 kDa), ECD showed that multiple molecular ions (+98, +178, etc.) are due to phosphate noncovalent adducts, and MS/MS pinpointed the overall mass discrepancy of 135 Da to removal of the initiation Met (131 Da) and to formation of disulfide bonds (2 x 2 Da) at C32-C49 and C143-C147, although 10 S-S positions were possible. In contrast, "bottom up" proteolysis characterization of the CoaBC and the P4H proteins was relatively unsuccessful. The addition of ECD substantially increases the capabilities of top down FTMS for the detailed structural characterization of large proteins.