Tomas Baer

Paul Scherrer Institut, Aargau, Switzerland

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Publications (188)511.86 Total impact

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    ABSTRACT: The applicability of the extended kinetic method (EKM) to determine the gas phase acidities (GA) of different deprotonable groups within the same molecule was tested by measuring the acidities of cinnamic, coumaric, and caffeic acids. These molecules differ not only in the number of acidic groups but in their nature, intramolecular distances, and calculated GAs. In order to determine independently the GA of groups within the same molecule using the EKM, it is necessary to selectively prepare pure forms of the hydrogen-bound heterodimer. In this work, the selectivity was achieved by the use of solvents of different vapor pressure (water and acetonitrile), as well as by variation of the drying temperature in the ESI source, which affected the production of heterodimers with different solvation energies and gas-phase dissociation energies. A particularly surprising finding is that the calculated solvation enthalpies of water and the aprotic acetonitrile are essentially identical, and that the different gas-phase products generated are apparently the result of their different vapor pressures, which affects the drying mechanism. This approach for the selective preparation of heterodimers, which is based on the energetics, appears to be quite general and should prove useful for other studies that require the selective production of heterodimers in ESI sources. The experimental results were supported by density functional theory (DFT) calculations of both gas-phase and solvated species. The experimental thermochemical parameters (deprotonation ΔG, ΔH, and ΔS) are in good agreement with the calculated values for the monofunctional cinnamic acid, as well as the multifunctional coumaric and caffeic acids. The measured GA for cinnamic acid is 334.5 ± 2.0 kcal/mol. The measured acidities for the COOH and OH groups of coumaric and caffeic acids are 332.7 ± 2.0, 318.7 ± 2.1, 332.2 ± 2.0, and 317.3 ± 2.2 kcal/mol, respectively.
    Journal of the American Chemical Society 06/2013; · 11.44 Impact Factor
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    ABSTRACT: The H/D loss and CH(3)/CD(3) loss reactions from energy selected ethanol isotopologue ions C(2)H(5)OH(+), C(2)D(5)OD(+), CD(3)CH(2)OH(+), and CH(3)CD(2)OH(+) have been studied by imaging threshold photoelectron photoion coincidence (iPEPICO) spectroscopy. In the lowest energy dissociation channel, the α-carbon loses a hydrogen or a deuterium atom. Asymmetry in the daughter ion time-of-flight (TOF) peaks, an ab initio study of the reaction rates, and shifts in the phenomenological onsets between isotopologues revealed that H/D loss is slow at its onset. Tunneling through a reverse barrier along the reaction coordinate was found to play a significant role. Modeling the data with an Eckart barrier suggests that H loss from light ethanol ions proceeds via a reverse barrier of 151 meV, which agrees very well with the ab initio result of 155 meV. The higher energy methyl loss channel appears at its thermochemical threshold, but the branching ratios for methyl and H loss as a function of the ion internal energy are not entirely consistent with statistical theory. The methyl-loss signal cannot completely outcompete the hydrogen atom loss process. The shape of the photoelectron spectrum as well as our calculations indicate that the lowest energy ethanol ion structure lies considerably below the reported IE of 10.48 eV. Franck-Condon factors are favorable for ionization to a metastable ion state, which can rearrange to a more stable equilibrium structure. Combining theoretical results with previous experimental ones yields a revised ethanol adiabatic ionization energy of 10.37 eV. This applies to all isotopologues, as the isotope effect on the ionization energy is not more than a few meV.
    Physical Chemistry Chemical Physics 10/2012; · 4.20 Impact Factor
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    ABSTRACT: Threshold photoelectron photoion coincidence (TPEPICO) has been used to study the sequential photodissociation reaction of internal energy selected 1,2-diiodoethane cations: C(2)H(4)I(2)(+) → C(2)H(4)I(+) + I → C(2)H(3)(+) + I + HI. In the first I-loss reaction, the excess energy is partitioned between the internal energy of the fragment ion C(2)H(4)I(+) and the translational energy. The breakdown diagram of C(2)H(4)I(+) to C(2)H(3)(+), i.e., the fractional ion abundances below and above the second dissociation barrier as a function of the photon energy, yields the internal energy distribution of the first daughter, whereas the time-of-flight peak widths yield the released translational energy in the laboratory frame directly. Both methods indicate that the kinetic energy release in the I-loss step is inconsistent with the phase space theory (PST) predicted two translational degrees of freedom, but is well-described assuming only one translational degree of freedom. Reaction path calculations partly confirm this and show that the reaction coordinate changes character in the dissociation, and it is, thus, highly anisotropic. For comparison, data for the dissociative photoionization of 1,3-diiodopropane are also presented and discussed. Here, the reaction coordinate is expected to be more isotropic, and indeed the two degrees of freedom assumption holds. Characterizing kinetic energy release distributions beyond PST is crucial in deriving accurate dissociative photoionization onset energies in sequential reactions. On the basis of both experimental and theoretical grounds, we also suggest a significant revision of the 298 K heat of formation of 1,2-C(2)H(4)I(2)(g) to 64.5 ± 2.5 kJ mol(-1) and that of CH(2)I(2)(g) to 113.5 ± 2 kJ mol(-1) at 298 K.
    The Journal of Physical Chemistry A 02/2012; 116(11):2833-44. · 2.77 Impact Factor
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    ABSTRACT: The dissociation dynamics of Sn(CH(3))(4)(+), Sn(CH(3))(3)Cl(+), and Sn(CH(3))(3)Br(+) were investigated by threshold photoelectron photoion spectrometry using an electron imaging apparatus (iPEPICO) at the Swiss Light Source. The tetramethyltin ion was found to dissociate via Sn(CH(3))(4)(+) → Sn(CH(3))(3)(+) + CH(3) → Sn(CH(3))(2)(+) + 2CH(3), while the trimethyltin halide ions dissociated via methyl loss at low energies, and a competitive halogen loss at somewhat higher energies. The 0 K methyl loss onset for the three ions was found to be 9.410 ± 0.020 eV, 10.058 ± 0.020 eV, and 9.961 ± 0.020 eV, respectively. Statistical theory could not reproduce the observed onsets for the halogen loss steps in the halotrimethyltin ions. The halide loss signal as a function energy mimicked the excited state threshold photoelectron spectrum, from which we conclude that the halide loss from these ions takes place on an isolated excited state potential energy surface, which we describe by time dependent density functional calculations. The sequential loss of a second methyl group in the Sn(CH(3))(4)(+) ion, observed at about 3 eV higher energies than the first one, is also partially non-statistical. The derived product energy distribution resulting from the loss of the first methyl group is two-component with about 50% being statistical and the remainder associated with high translational energy products that peak at 2 eV. Time dependent DFT calculations show that a dissociative ͠B state lies in the vicinity of the experimental measurements. We thus propose that 50% of the Sn(CH(3))(4)(+) ions produced in this energy range internally convert to the ͠X state, on which they dissociate statistically, while the remainder dissociate directly from the repulsive ͠B state leading to high kinetic energy products.
    Physical Chemistry Chemical Physics 09/2011; 13(39):17791-801. · 4.20 Impact Factor
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    ABSTRACT: The dissociative photoionization of tetramethyltin (Me₄Sn) and hexamethylditin (Me₆Sn₂) has been investigated by threshold photoelectron-photoion coincidence (TPEPICO). Ions are energy-selected, and their 0 K dissociation onsets are measured by monitoring the mass spectra as a function of ion internal energy. Me₄Sn(+) dissociates rapidly by methyl loss, with a 0 K onset of E₀ = 9.382 ± 0.020 eV. The hexamethylditin ion dissociates slowly on the time scale of the experiment (i.e., during the 40 μs flight time to the detector) so that dissociation rate constants are measured as a function of the ion energy. RRKM and the simplified statistical adiabatic channel model (SSACM) are used to extrapolate the measured rate constants for methyl and Me₃Sn(•) loss to their 0 K dissociation onsets, which were found to be 8.986 ± 0.050 and 9.153 ± 0.075 eV, respectively. Updated values for the heats of formation of the neutral Me₄Sn and Me₆Sn₂ are used to derive the following 298.15 K gas-phase standard heats of formation, in kJ·mol⁻¹: Δ(f)H(m)(o)(Me₃Sn(+),g) = 746.3 ± 2.9; Δ(f)H(m)(o)(Me₅Sn₂(+),g) = 705.1 ± 7.5; Δ(f)H(m)(o)(Me₃Sn(•),g) = 116.6 ± 9.7; Δ(f)H(m)(o)(Me₂Sn,g) = 123.0 ± 16.5; Δ(f)H(m)(o)(MeSn(+),g) = 877.8 ± 16.4. These energetic values also lead to the following 298.15 K bond dissociation enthalpies, in kJ·mol⁻¹: BDE(Me₃Sn-Me) = 284.1 ± 9.9; BDE(Me₃Sn-SnMe₃) = 252.6 ± 14.8.
    The Journal of Physical Chemistry A 02/2011; 115(4):402-9. · 2.77 Impact Factor
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    ABSTRACT: The dissociative photoionization of 1,1-C(2)H(2)Cl(2), (E)-1,2-C(2)H(2)Cl(2), and (Z)-1,2-C(2)H(2)Cl(2) has been investigated at high energy and mass resolution using the imaging photoelectron photoion coincidence instrument at the Swiss Light Source. The asymmetric Cl-atom loss ion time-of-flight distributions were fitted to obtain the dissociation rates in the 10(3) s(-1) < k < 10(7) s(-1) range as a function of the ion internal energy. The results, supported by ab initio calculations, show that all three ions dissociate to the same C(2v) symmetry ClC═CH(2)(+) product ion. The 0 K onset energies thus establish the relative heats of formation of the neutral isomers, that is, the isomerization energies. The experimental rate constants, k(E), as well as ab initio calculations indicate an early isomerization transition state and no overall reverse barrier to dissociation. The major high energy channels are the parallel HCl loss and the sequential ClC═CH(2)(+) → HCCH(+) + Cl process, the latter in competition with a ClC═CH(2)(+) → ClCCH(+) + H reaction. A parallel C(2)H(2)Cl(2)(+) → C(2)HCl(2)(+) + H channel also weakly asserts itself. The 0 K onset energy for the sequential Cl loss reaction suggests no barrier to the production of the most stable acetylene ion product; thus the sequential Cl-atom loss is preceded by a ClC═CH(2)(+) → HC(Cl)CH(+) reorganization step with a barrier lower than that of the second Cl-atom loss. The breakdown diagram corresponding to this sequential dissociation reveals the internal energy distribution of the first C(2)H(2)Cl(+) daughter ion, which is determined by the kinetic energy release in the first, Cl loss reaction at high excess energies. At low kinetic energy release, this distribution corresponds to the predicted two translational degrees of freedom, whereas at higher energies, the excess energy partitioning is characteristic of only one translational degree of freedom. New Δ(f)H(o)(298K) of 3.7, 2.5, and 0.2 ± 1.75 kJ mol(-1) are proposed for 1,1-C(2)H(2)Cl(2), (E)-1,2-C(2)H(2)Cl(2), and (Z)-1,2-C(2)H(2)Cl(2), respectively, and the proton affinity of ClCCH is found to be 708.6 ± 2.5 kJ mol(-1).
    The Journal of Physical Chemistry A 02/2011; 115(5):726-34. · 2.77 Impact Factor
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    ABSTRACT: Metallocene ions (Cp(2)M(+), M = Cr, Co, Ni) were studied by threshold photoelectron photoion coincidence spectroscopy (TPEPICO) to investigate the mechanism, energetics, and kinetics of the ionic dissociation processes. The examined energy-selected Cp(2)M(+) ions fragment by losing the neutral cyclopentadienyl ligand. In addition, CH and C(2)H(2) losses appear as minor channels, while the cobaltocene ion also loses an H atom. A possible isomerization pathway has also been observed for Cp(2)Ni(+), yielding a complex with pentafulvalene (C(10)H(8)) with a loss of H(2). In order to determine the 0 K appearance energies for the CpM(+) fragment ions, the asymmetric time-of-flight peak shapes and the breakdown diagrams of the energy-selected metallocene ions were modeled by both the rigid activated complex (RAC) Rice-Ramsperger-Kassel-Marcus (RRKM) theory and the simplified statistical adiabatic channel model (SSACM). The following appearance energies were obtained with SSACM, which is more reliable for loose transition states: 10.57 ± 0.14, 11.01 ± 0.13, and 10.18 ± 0.13 eV for M = Cr, Co, and Ni, respectively. These values combined with the corresponding adiabatic ionization energies yield M-Cp bond dissociation energies in Cp(2)M(+) ions of 5.04 ± 0.16, 5.77 ± 0.15, and 3.96 ± 0.15 eV. Density functional calculations at the B3LYP/6-311G(d,p) level of theory were used to determine the structures of these complexes and to provide parameters necessary for the analysis of the experimental data. The trends in the M-Cp bond energies can be related to the electronic structures of the metallocene ions based on a simple molecular orbital picture.
    Journal of the American Chemical Society 12/2010; 132(50):17795-803. · 11.44 Impact Factor
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    ABSTRACT: Threshold photoelectron photoion coincidence has been used to prepare selected internal energy distributions of nitrosobenzene ions [C(6)H(5)NO(+)]. Dissociation to C(6)H(5)(+) + NO products was measured over a range of internal energies and rate constants from 10(3) to 10(7) s(-1) and fitted with the statistical theory of unimolecular decay. A 0 K dissociative photoionization onset energy of 10.607 ± 0.020 eV was derived by using the simplified statistical adiabatic channel model. The thermochemical network of Active Thermochemical Tables (ATcT) was expanded to include phenyl and phenylium, as well as nitrosobenzene. The current ATcT heats of formation of these three species at 0 K (298.15 K) are 350.6 (337.3) ± 0.6, 1148.7 (1136.8) ± 1.0, and 215.6 (198.6) ± 1.5 kJ mol(-1), respectively. The resulting adiabatic ionization energy of phenyl is 8.272 ± 0.010 eV. The new ATcT thermochemistry for phenyl entails a 0 K (298.15 K) C-H bond dissociation enthalpy of benzene of 465.9 (472.1) ± 0.6 kJ mol(-1). Several related thermochemical quantities from ATcT, including the current enthalpies of formation of benzene, monohalobenzenes, and their ions, as well as interim ATcT values for the constituent atoms, are also given.
    The Journal of Physical Chemistry A 12/2010; 114(50):13134-45. · 2.77 Impact Factor
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    ABSTRACT: A computer program has been developed to model and analyze the data from photoelectron photoion coincidence (PEPICO) spectroscopy experiments. This code has been used during the past 12 years to extract thermochemical and kinetics information for almost a hundred systems, and the results have been published in over forty papers. It models the dissociative photoionization process in the threshold PEPICO experiment by calculating the thermal energy distribution of the neutral molecule, the energy distribution of the molecular ion as a function of the photon energy, and the resolution of the experiment. Parallel or consecutive dissociation paths of the molecular ion and also of the resulting fragment ions are modeled to reproduce the experimental breakdown curves and time-of-flight distributions. The latter are used to extract the experimental dissociation rates. For slow dissociations, either the quasi-exponential fragment peak shapes or, when the mass resolution is insufficient to model the peak shapes explicitly, the center of mass of the peaks can be used to obtain the rate constants. The internal energy distribution of the fragment ions is calculated from the densities of states using the microcanonical formalism to describe consecutive dissociations. Dissociation rates can be calculated by the RRKM, SSACM or VTST rate theories, and can include tunneling effects, as well. Isomerization of the dissociating ions can also be considered using analytical formulae for the dissociation rates either from the original or the isomer ions. The program can optimize the various input parameters to find a good fit to the experimental data, using the downhill simplex algorithm.
    Biological Mass Spectrometry 11/2010; 45(11):1233-45. · 2.71 Impact Factor
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    ABSTRACT: The dissociation dynamics of energy selected i-C(3)H(7)X (X = H, Cl, Br, and I) ions have been investigated by imaging photoelectron-photoion coincidence (iPEPICO) spectroscopy using synchrotron radiation from the X04DB VUV beamline in the Swiss Light Source of the Paul Scherrer Institut. The 0 K dissociation energy (E(0)) for i-C(3)H(8) was determined to be 11.624 ± 0.002 eV. This leads to a 298 K isopropyl ion heat of formation of 805.9 ± 0.5 kJ mol(-1). The Δ(f)H(298K)°(i-C(3)H(7)(+)) combined with the measured 0 K onsets for i-C(3)H(7)(+) formation from isopropyl chloride (11.065 ± 0.004 eV), isopropyl bromide (10.454 ± 0.008 eV), and isopropyl iodide (9.812 ± 0.008 eV) yields the 298 K isopropyl chloride, bromide, and iodide heats of formation of -145.7 ± 0.7, -95.6 ± 0.9, and -38.5 ± 0.9 kJ mol(-1), respectively. These values provide a significant correction to literature values and reduce the error limits. Finally, the new i-C(3)H(7)(+) heat of formation leads to a predicted adiabatic ionization energy for the isopropyl radical of 7.430 ± 0.012 eV and a 298 K proton affinity for propene of 744.1 ± 0.8 kJ mol(-1).
    The Journal of Physical Chemistry A 10/2010; 114(42):11285-91. · 2.77 Impact Factor
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    ABSTRACT: The dissociation dynamics of gas phase formic acid ions (HCOOH(+), DCOOD(+), HCOOD(+), DCOOH(+)) are investigated by threshold photoelectron-photoion coincidence (TPEPICO) spectroscopy and high level ab initio calculations. The slow rate constants for this seemingly simple H loss reaction and the large onset energy shifts due to isotopic substitution point to a substantial exit barrier through which the H or D atoms tunnel. Modeling of the HCOOH(+) experimental data using RRKM theory with tunneling through an Eckart potential are best fitted with a barrier of about 17 kJ mol(-1). High level ab initio calculations support the experimental findings with a computed barrier of 15.9 kJ mol(-1), which is associated with the substantial geometry change between the product HOCO(+) cation and the corresponding HCOOH(+) molecular ion. Because of this exit channel barrier, the formic acid ion dissociation does not provide a route for determination of the HOCO(+) heat of formation. Rather, the most accurate value comes from the calculations employing the high accuracy extrapolated ab initio thermochemistry (HEAT) scheme, which yields a Δ(f)H(o)(0K)[HOCO(+)] = 600.3 ± 1.0 kJ mol(-1) (Δ(f)H(o)(298K)[HOCO(+)] = 597.3 ± 1.0 kJ mol(-1)). The calculated proton affinity of CO(2) is thus 534.7 ± 1.0 kJ mol(-1) at 0 K and 539.3 ± 1.0 kJ mol(-1) at 298.15 K.
    The Journal of Physical Chemistry A 09/2010; 114(37):10016-23. · 2.77 Impact Factor
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    ABSTRACT: The dissociation of energy-selected acetic acid ions (CH3COOH+) has been investigated by Threshold Photoelectron–photoion Coincidence (TPEPICO) spectroscopy. The lowest energy dissociation pathway for CH3COOH+ is OH loss, and the 0K onset (E0) of this process is measured to be 11.641±0.008eV. The reaction rate for this step is instantaneous on the time-scale of the experiment so that no rate theory is required to extract its onset. Because the heats of formation of acetic acid and the OH radical are known to an accuracy of ±0.5kJ/mol, we can use the OH loss onset to obtain an accurate heat of formation of the acetyl ion, ΔfH°298K[CH3CO+]=658.5±1.0kJ/mol, which agrees to within 0.9kJ/mol with an earlier determination based on CH3 loss from the acetone ion. At higher energies, the acetic acid ion loses CH3 to form the HOCO+ ion in competition with the lower energy OH-loss. Two versions of the statistical reaction rate theory failed to reproduce the branching ratios for these higher energy parallel reactions. Because of the magnitude of this disagreement, we conclude that the acetic ion may dissociate non-statistically at these higher energies.
    International Journal of Mass Spectrometry 07/2010; 294(2):88-92. · 2.23 Impact Factor
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    Journal of the American Society for Mass Spectrometry 05/2010; 21(5):i-iii. · 3.59 Impact Factor
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    Tomas Baer, Robert C Dunbar
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    ABSTRACT: The ASMS conference on ion spectroscopy brought together at Asilomar on October 16-20, 2009 a large group of mass spectrometrists working in the area of ion spectroscopy. In this introduction to the field, we provide a brief history, its current state, and where it is going. Ion spectroscopy of intermediate size molecules began with photoelectron spectroscopy in the 1960s, while electronic spectroscopy of ions using the photodissociation "action spectroscopic" mode became active in the next decade. These approaches remained for many years the main source of information about ionization energies, electronic states, and electronic transitions of ions. In recent years, high-resolution laser techniques coupled with pulsed field ionization and sample cooling in molecular beams have provided high precision ionization energies and vibrational frequencies of small to intermediate sized molecules, including a number of radicals. More recently, optical parametric oscillator (OPO) IR lasers and free electron lasers have been developed and employed to record the IR spectra of molecular ions in either molecular beams or ion traps. These results, in combination with theoretical ab initio molecular orbital (MO) methods, are providing unprecedented structural and energetic information about gas-phase ions.
    Journal of the American Society for Mass Spectrometry 02/2010; 21(5):681-93. · 3.59 Impact Factor
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    ABSTRACT: The dissociations of energy-selected di-t-butyl peroxide and di-t-butyl diazene ions have been studied by threshold photoelectron-photoion coincidence (TPEPICO) spectroscopy. Di-t-butyl peroxide ions dissociate via two parallel channels: (1) methyl loss at a 0 K onset (E0) of 9.58 +/- 0.04 eV followed by a sequential dissociation of the daughter ion to produce C4H9O+ and acetone; and (2) the dominant dissociation channel, producing t-butyl ion and t-butyl peroxy radical at an E0 of 9.758 +/- 0.020 eV. Di-t-butyl diazene ions dissociate through three parallel channels: (1) a rearrangement to form isobutene ion; (2) C-N bond cleavage with the charge staying on the t-butyl diazyl species (E0 = 8.069 +/- 0.050 eV); and (3) C-N bond cleavage with the charge instead on the t-butyl (E0 = 8.122 +/- 0.050 eV); the coproduct for this latter channel is a weakly, or possibly unbound, N2...t-butyl structure. Both the peroxide and diazene ion dissociations produce metastable daughters, and the dissociation rates are modeled with two rate theories: the Rice-Ramsperger-Kassel-Marcus (RRKM) theory and a simplified version of the statistical adiabatic channel model (SSACM). Due to a large kinetic shift, RRKM incorrectly models the peroxide ion rate curve. Using SSACM, the heat of formation of t-butyl peroxy radical is determined to be DeltaH0Kdegrees = - 81.1 +/- 3.9 kJ mol-1, and, using B3LYP/6-311++G(d,p) thermal energy, DeltaH298Kdegrees = - 109.7 +/- 3.9 kJ mol-1. Due to a competitive shift of the higher energy channel onsets, RRKM also incorrectly models the diazene rate curves. The 298 K heat of formation of the t-butyl diazyl ion, which is bound by 14 kJ mol-1, is determined to be 701.2 +/- 5.9 kJ mol-1.
    The Journal of Physical Chemistry A 01/2010; 114(1):232-40. · 2.77 Impact Factor
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    ABSTRACT: The dissociative photoionization onset energy of the CH(3)I --> CH(3)(+) + I reaction was studied at the vacuum ultraviolet (VUV) beamline of the Swiss Light Source (SLS) using a new imaging photoelectron photoion coincidence (iPEPICO) apparatus operating with a photon resolution of 2 meV and a threshold electron kinetic energy resolution of about 1 meV. Three previous attempts at establishing this value accurately, namely a pulsed field ionization (PFI)-PEPICO measurement, ab initio calculations and a mass-analyzed threshold ionization (MATI) experiment, in which the onset energy was bracketed by state-selected excitation to vibrationally excited (2)A(1) A states of the parent ion, have yielded contradictory results. It is shown that dimers and adducts formed in the supersonic molecular beam affected the PFI-PEPICO onset energy. The room temperature iPEPICO experiment yields an accurate 0 K onset of 12.248 +/- 0.003 eV, from which we derive a Delta(f)H(o)(298 K)(CH(3)I) = 15.23 +/- 0.3 kJ mol(-1), and the C-I bond energy in CH(3)I is 232.4 +/- 0.4 kJ mol(-1). The room temperature breakdown diagram shows a fine structure that corresponds to the threshold photoelectron spectrum (TPES) of the A state. Low internal energy neutrals seem to be preferentially ionized in the A state when compared with the X state, and A state peaks in the TPES are Stark-shifted as a function of the DC field, whereas the dissociative photoionization of X state ions is not affected. This suggests that there are different competing mechanisms at play to produce ions in the A state vs. ions in the X state. The competition between field ionization and autoionization in CH(3)I is compared with that in Ar, N(2) and in the H-atom loss energy region in CH(4)(+). The binding energies of the neutral and ionic Ar-CH(3)I clusters were found to be 26 and 66 meV, respectively.
    Physical Chemistry Chemical Physics 12/2009; 11(46):11013-21. · 4.20 Impact Factor
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    ABSTRACT: The dissociation dynamics of energy selected neopentane, t-butyl iodide, and t-butyl hydroperoxide ions have been investigated by threshold photoelectron-photoion coincidence (TPEPICO) spectrometry. Although the methyl loss reaction from neopentane ions producing the t-butyl ion is in competition with a lower energy methane loss channel, modeling these two channels with the statistical theory of unimolecular decay provides a 0 K dissociation onset for methyl loss of 10.564 +/- 0.025 eV. This leads to a 298 K t-butyl ion heat of formation of 714.3 +/- 2.5 kJ x mol(-1), which is some 3 kJ x mol(-1) higher than the previously accepted value. The Delta(f)H degrees (298K)(t-C(4)H(9)(+)) combined with the measured 0 K onsets for t-C(4)H(9)(+) formation from t-butyl iodide (9.170 +/- 0.007 eV) and from t-butyl hydroperoxide (9.904 +/- 0.012 eV), yields 298 K t-butyl iodide and t-butyl hydroperoxide heats of formation of -68.5 +/- 2.6 kJ x mol(-1) and -233.2 +/- 2.8 kJ x mol(-1), respectively. Finally, the new t-C(4)H(9)(+) heat of formation leads to a predicted adiabatic ionization energy for the t-butyl radical of 6.86 +/- 0.20 eV, and a 298 K proton affinity for isobutene of 798.8 +/- 2.5 kJ x mol(-1). The predicted ionization energy exceeds all measured values by 0.10 eV.
    The Journal of Physical Chemistry A 10/2009; 114(2):804-10. · 2.77 Impact Factor
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    ABSTRACT: The 0 K onset of C(3)H(6) --> C(3)H(5)(+) + H(*) is measured by threshold photoelectron-photoion coincidence (TPEPICO) spectroscopy. From the onset (11.898 +/- 0.025 eV) the heat of formation of the allyl ion (CH(2)CHCH(2)(+)) is determined to be DeltaH degrees (f,0K) = 967.2; DeltaH degrees (f,298K) = 955.4 +/- 2.5 kJ mol(-1). The value is significantly more positive than prior determinations, and resolves a discrepancy between measurements of the allyl radical and allyl ion heats of formation and recent highly precise measurements of the allyl radical adiabatic ionization energy. The new allyl ion heat of formation leads to a new proton affinity for propadiene (allene) of 765.0 +/- 2.6 kJ mol(-1). An attempt is made to determine the CH(3)CCH(2)(+) heat of formation by measuring the 0 K onset of 2-ClC(3)H(5) --> C(3)H(5)(+) + Cl(*). However, C(3)H(5)(+) appears at too low an energy to be the higher energy CH(3)CCH(2)(+) structure. Rather, 2-ClC(3)H(5)(+) undergoes a concerted hydrogen transfer and Cl-loss via an intramolecular S(N)2 like mechanism to produce the allyl ion. The 0 K onset of 3-ClC(3)H(5) --> C(3)H(5)(+) + Cl(*) (11.108 +/- 0.010 eV) is measured to determine the 3-ClC(3)H(5) heat of formation (DeltaH degrees (f,0K) = 14.9; DeltaH degrees (f,298K) = 1.1 +/- 2.7 kJ mol(-1)). 3-ClC(3)H(5)(+) is suggested to readily isomerize to trans 1-ClC(3)H(5)(+) prior to dissociation.
    The Journal of Physical Chemistry A 09/2009; 113(40):10710-6. · 2.77 Impact Factor
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    ABSTRACT: The 0 K onsets (E(0)) of a series of trichlorosilane derivatives SiCl(3)R --> SiCl(3)(+)+ R(*) (R = Cl, H, CH(3), C(2)H(5), C(2)H(3), CH(2)Cl, SiCl(3)) are measured by threshold photoelectron-photoion coincidence spectroscopy. The well-known heat of formation of SiCl(4) is used as an anchor to determine the heat of formation of SiCl(3)(+), which is, in turn, used as an anchor to determine the heats of formation of the other alkyltrichlorosilanes investigated. A series of isodesmic reactions at the G3 and CBS-QB3 levels are shown to accurately reproduce the experimental heats of formation, and this scheme is used to calculate the heat of formation of Si(2)Cl(6), from which the measured E(0) determines the SiCl(3)(*) heat of formation. The measured values then determine the IE of SiCl(3)(*) along with the Si-R bond dissociation enthalpies of the six neutral species investigated. The experimental heats of formation are also used in a series of isodesmic reaction calculations to determine the heats of formation of SiH(3)R (R = H, CH(3), C(2)H(5), C(2)H(3), CH(2)Cl, SiCl(3)).
    The Journal of Physical Chemistry A 08/2009; 113(34):9458-66. · 2.77 Impact Factor
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    ABSTRACT: Threshold photoelectron photoion coincidence spectroscopy is used to study the dissociation of energy-selected X(CH(3))(3)(+) ions (X = As, Sb, Bi) by methyl loss, the only process observed up to 2 eV above the ionization energy. The ion time-of-flight distributions and the breakdown diagrams are analyzed in terms of the statistical RRKM theory to obtain accurate ionic dissociation energies. These experiments complement previous studies on analogous trimethyl compounds of the N group where X = N and P. However, trimethylamine was observed to lose only an H atom, whereas trimethylphosphine was shown to lose methyl radical, H atom, and, to a lesser extent, methane in parallel dissociation reactions. Both kinetic and thermodynamic arguments are needed to explain these trends. The methyl radical loss has two channels: either a H transfer to the central atom, followed by CH(3) loss, or a direct homolytic bond cleavage. However, the H transfer channel is blocked in trimethylamine by an H loss channel with an earlier onset, and, thus, the methyl loss is not observed. Bond energies are defined based on ab initio reaction energies and show that the main thermodynamic reason behind the trends in the energetics is the significantly weakening C=X double bond in the ion in the N --> As direction. The first adiabatic ionization energies of Sb(CH(3))(3) and Bi(CH(3))(3) have also been measured by ultraviolet photoelectron spectroscopy to be 8.02 +/- 0.05 and 8.08 +/- 0.05 eV, respectively.
    The Journal of Physical Chemistry A 06/2009; 113(28):8091-8. · 2.77 Impact Factor

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  • 2009–2012
    • Paul Scherrer Institut
      • Research Department General Energy (ENE)
      Aargau, Switzerland
  • 1973–2012
    • University of North Carolina at Chapel Hill
      • Department of Chemistry
      Chapel Hill, NC, United States
  • 2011
    • Spanish National Research Council
      • Institute of Physical Chemistry "Rocasolano"
      Madrid, Madrid, Spain
    • Institute of Physical Chemistry Rocasolano
      Madrid, Madrid, Spain
  • 2010
    • University of the Pacific (California - USA)
      • Department of Chemistry
      Stockton, California, United States
  • 2005–2010
    • Eötvös Loránd University
      • Institute of Chemistry
      Budapest, Budapest fovaros, Hungary
    • Université Paris-Est Marne-la-Vallée
      Champs, Île-de-France, France
  • 2002
    • University of Science and Technology of China
      Luchow, Anhui Sheng, China
  • 2001
    • Freie Universität Berlin
      • Division of Physical and Theoretical Chemistry
      Berlin, Land Berlin, Germany
  • 1991
    • Georg-August-Universität Göttingen
      • Institute of Physical Chemistry
      Göttingen, Lower Saxony, Germany