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ABSTRACT: The gas-phase ion-molecule reactions of 1,1,3,3-tetrafluorodimethyl ether and water have been examined using Fourier transform ion cyclotron resonance mass spectrometry, infrared multiphoton dissociation (IRMPD) spectroscopy, and ab initio molecular orbital calculations. This reaction sequence leads to the efficient bimolecular production of the proton-bound dimer of water (H5O2+). Evidence for the dominant mechanistic pathway involving the reaction of CF2H-O=CHF+, an ion of m/z 99, with water is presented. The primary channel occurs via nucleophilic attack of water on the ion of m/z 99 (CF2H-O=CHF+), to lose formyl fluoride and yield-protonated difluoromethanol (m/z 69). Association of a second water molecule with protonated difluoromethanol generates a reactive intermediate that decomposes via a 1,4-elimination to release hydrogen fluoride and yield the proton-bound dimer of water and formyl fluoride (m/z 67). Last, the elimination of formyl fluoride occurs by the association of a third water molecule to produce H5O2+ (m/z 37). The most probable isomeric forms of the ions with m/z 99 and 69 were found using IRMPD spectroscopy and electronic structure theory calculations. Thermochemical information for reactant, transition state, and product species was obtained using MP2(full)/6-311+G**//6-31G* level of theory.
The Journal of Physical Chemistry A 10/2007; 111(36):8792-802. · 2.95 Impact Factor
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ABSTRACT: Density functional theory computations and pulsed-ionization high-pressure mass spectrometry experiments have been used to explore the potential energy surfaces for gas-phase S(N)2 reactions between halide ions and trifluoromethyl halides, X(-) + CF(3)Y --> Y(-) + CF(3)X. Structures of neutrals, ion-molecule complexes, and transition states show the possibility of two mechanisms: back- and front-side attack. From pulsed-ionization high-pressure mass spectrometry, enthalpy and entropy changes for the equilibrium clustering reactions for the formation of Cl(-)(BrCF(3)) (-16.5 +/- 0.2 kcal mol(-1) and -24.5 +/- 1 cal mol(-1) K(-1)), Cl(-)(ICF(3)) (-23.6 +/- 0.2 kcal mol(-1)), and Br(-)(BrCF(3)) (-13.9 +/- 0.2 kcal mol(-1) and -22.2 +/- 1 cal mol(-1) K(-1)) have been determined. These are in good to excellent agreement with computations at the B3LYP/6-311+G(3df)//B3LYP/6-311+G(d) level of theory. It is shown that complex formation takes place by a front-side attack complex, while the lowest energy S(N)2 reaction proceeds through a back-side attack transition state. This latter mechanism involves a potential energy profile which closely resembles a condensed phase S(N)2 reaction energy profile. It is also shown that the Cl(-) + CF(3)Br --> Br(-) + CF(3)Cl S(N)2 reaction can be interpreted using Marcus theory, in which case the reaction is described as being initiated by electron transfer. A potential energy surface at the B3LYP/6-311+G(d) level of theory confirms that the F(-) + CF(3)Br --> Br(-) + CF(4) S(N)2 reaction proceeds through a Walden inversion transition state.
The Journal of Physical Chemistry A 02/2006; 110(4):1350-63. · 2.95 Impact Factor
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ABSTRACT: Mass spectrometry-based methods have been employed in order to study the reactions of non- (h(6)/h(6)), half (d(6)/h(6)), and fully (d(6)/d(6)) deuterium labeled protonated dimers of acetone in the gas phase. Neither kinetic nor thermodynamic isotope effects were found. From MIKES experiments (both spontaneous and collision-induced dissociations), it was found that the relative ion yield (m/z 65 vs m/z 59) from the dissociation reaction of half deuterium labeled (d(6)/h(6)) protonated dimer of acetone is dependent on the internal energy. A relative ion yield (m/z 65 vs m/z 59) close to unity is observed for cold, nonactivated, metastable ions, whereas the ion yield is observed to increase (favoring m/z 65) when the pressure of the collision gas is increased. This is in striking contrast to what would be expected if a kinetic isotope effect were present. A combined study of the kinetics and the thermodynamics of the association reaction between acetone and protonated acetone implicates the presence of at least two isomeric adducts. We have employed G3(MP2) theory to map the potential energy surface leading from the reactants, acetone and protonated acetone, to the various isomeric adducts. The proton-bound dimer of acetone was found to be the lowest-energy isomer, and protonated diacetone alcohol the next lowest-energy isomer. Protonated diacetone alcohol, even though it is an isomer hidden behind many barriers, can possibly account for the observed relative ion yield and its dependence on the mode of activation.
Journal of Mass Spectrometry 09/2005; 40(8):1076-87. · 3.27 Impact Factor
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04/2002;
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ABSTRACT: Fourier transform ion cyclotron resonance mass spectrometry has been used to study the temperature and deuterium isotope effects on the methyl cation transfer reaction between protonated dimethyl ether and dimethyl ether to produce trimethyloxonium cation and methanol. From the temperature dependence of this bimolecular reaction it was possible to obtain thermodynamic information concerning the energy barrier for methyl cation transfer for the first time. From the slope of an Arrhenius plot, a value for DeltaH(++) of -1.1 +/- 1.2 kJ mol(-1) was obtained, while from the intercept a value for DeltaS(++) of -116 +/- 15 J K(-1) mol(-1) was derived. This yields a DeltaG(++)(298) value of 33.7 +/- 2.1 kJ mol(-1). All thermodynamic values were in good agreement with ab initio calculations. Rate constant ratios for the unimolecular dissociation forming trimethyloxonium cation and the dissociation re-forming reactants were extracted from the apparent bimolecular rate constant. Attempts at modeling the temperature dependence and isotope effects of the unimolecular dissociation forming trimethyloxonium cation were also made.
Journal of the American Chemical Society 06/2001; 123(17):3980-5. · 9.91 Impact Factor
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08/2000;
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ABSTRACT: The gas-phase clustering equilibria of halide ions to a homologous series of alcohol molecules, X- + HOR X-(HOR) (X = F, Cl, Br, I; R = CH3, CH3CH2, (CH3)2CH, (CH3)3C), have been investigated using ab initio (MP2(full)) and density functional theory (B3LYP) computational methods. For both methods, extended basis sets, including diffuse and polarization functions for all atoms and anions, except I-, were used. For I- three different effective core potentials (ECP) were used to test their suitability for these systems. Comparing the Δ and Δ values obtained with various experimental data indicates that the MP2 and MP2//B3LYP methods perform best. Structural and spectroscopic features, as well as charge distributions, show interesting trends for the various X-(HOR) complexes, and the intrinsic contributions of the halide ions and the alcohol molecules to these trends are discussed. Finally, two-dimensional potential energy surface scans were performed for the X-(HOCH3) complexes at the MP2/6-311++G(d,p) level of theory. These surfaces reveal the asymmetric nature of the potential energy surface for the heavier halide ions, and the “floppy” nature of all the halide ion adducts.
07/2000;
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ABSTRACT: High-pressure mass spectrometric experiments and ab initio calculations have been carried out in order to establish a series of accurate gas-phase sodium ion affinities of organic molecules with a wide variety of functional groups. Ab initio calculations have also been performed on the sodium complexes of three amino acids: serine, cysteine, and proline. A systematic critical evaluation of experimental and computational literature results shows that a significant number require revision. Based on comparisons with accurate experimental measurements, the ab initio procedure used is shown to yield sodium ion affinities with an accuracy of ca. 1 kcal·mol-1. This enables the construction of the first reliable table of gas-phase Na+ affinities for organic and small biological molecules.
09/1999;
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ABSTRACT: Proton exchange reactions between acetate, n-butanoate, 2-ethylhexanoate, and n-decanoate were studied experimentally by the use of PHPMS and theoretically by the use of ab initio methods. The occurrence of a curvature in some of the van't Hoff plots suggests isomerization of at least one of the participants in the equilibrium. This isomerization is suggested to be an intramolecular solvation of the carboxylate anions via unconventional hydrogen bonding. These interactions are discussed in terms of charge distributions in the unfolded and folded conformers of the carboxylates. Thermochemical values for the intramolecular solvation were deconvoluted from the curved van't Hoff plots by a fitting procedure. The thermochemical data for the intramolecular solvation was used to calculate the conformer composition of the carboxylate anions. Various properties related to the intramolecular solvation of the carboxylate anions are discussed.
08/1999;
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ABSTRACT: Consecutive infrared multiphoton dissociations (IRMPD) may be observed in a Fourier transform ion cyclotron resonance mass spectrometer (FTICR). This is the IRMPD equivalent of previous MS(n)() experiments using CID. This work presents a versatile technique, using a bistable shutter to gate ON and OFF a continuous-wave (CW) CO(2) laser for multiple irradiation periods of 0.1-1000 s duration. Consecutive photodissociations, up to MS(4), are demonstrated for the proton-bound dimer of diethyl ether and the resulting fragment ions. The photoproducts are formed close to the center of the FTICR cell, resulting in high product ion recovery efficiency. This differs from CID products, which are formed throughout the FTICR cell causing reisolation/detection problems. The fragmentation resulting from the use of low-intensity, CW, infrared laser radiation is shown to be much more energy selective than CID. Photodissociation of C(2)H(5)OH(2)(+) ion produces the lowest energy product ion exclusively, even though the two product channels differ only by ∼5 kcal/mol. Low-energy CID, however, produces a mixture of C(2)H(5)(+) and H(3)O(+) products in the ratio of 1.3:1. Hence, the higher energy pathway (C(2)H(5)(+)) is substantially favored. The current results indicate that this IRMPD MS(n)() technique may be successfully applied to large biomolecules prepared by electrospray or MALDI.
Analytical Chemistry 12/1997; 69(23):4735-40. · 5.86 Impact Factor
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ABSTRACT: Pulsed ionization High-Pressure Mass Spectrometry (PHPMS) has been used to investigate the thermochemistry of the association reactions of t-C4H9+ with various neutral molecules in the gas phase. The behavior of the logarithm of the association equilibrium constant as a function of inverse reaction temperature (a van't Hoff plot) has been examined over a broad temperature range to yield accurate thermochemical data for the enthalpy and entropy of association. All of the systems investigated show either a pronounced break or a subtle curvature in the van't Hoff plot. This behavior is consistent with two coexisting isomeric forms of the association adduct, viz, (i) a low-temperature, covalently bound isomer characterized by larger −ΔH° and −ΔS° values and (ii) a high-temperature, electrostatically bound isomer of lower −ΔH° and −ΔS° values. Experimental thermochemical data for the association reactions are reported. From the low-temperature data, proton affinities (PA) of t-C4H9OCH3 (205.1 kcal/mol) and t-C4H9OC2H5 (214.0 kcal/mol) were determined. Ab initio calculations carried out in conjunction with the experimental study of some of these association reactions support the experimental results. The results obtained for t-C4H9+ onto (CH3)2O revealed the existence of two broad energy plateaus when attempting to optimize different electrostatically bound adduct structures. Structures corresponding to these energy plateaus are proposed to be possible transition states characterized by unconventional ionic hydrogen bonds. Theoretical thermochemical data for the association reactions investigated are reported.
03/1996;
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Journal of the American Chemical Society 02/1986; 108(3):528-9. · 9.91 Impact Factor
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ABSTRACT: The thermochemistry of the chloride ion clustering onto dimethyl ether, diethyl ether, and three fluorinated ethers (CH3OCF3, (CF2H)2O, and CF3OCF2H) under thermal equilibrium conditions has been determined using pulsed-ionization high pressure mass spectrometry. The standard enthalpy (ΔH°) and entropy change (ΔS°) values obtained indicate a variety of different types of bonding in these complexes. The mode of binding is mainly determined by the number of fluorine atoms present and by the substitution pattern. In addition ab initio computational methods have been used to obtain more insight into the structures and energetics. ΔH°298 value calculated at the MP2/[6-311 ++ G(3df,3pd)/6-311 + G(2df,p)]//MP2/[6-31 + G(d)/6-31G(d)] level of theory show excellent agreement with the experimentally obtained ΔH° values.
International Journal of Mass Spectrometry.
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ABSTRACT: The reaction between BH4- and CH2O has been investigated in the gas phase using ion cyclotron resonance spectroscopy. No hydride transfer from BH4- to the carbonyl group is observed, however a novel reaction between enolate ions and diborane has been observed. © 1984.
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ABSTRACT: The spontaneous dissociation of mixed proton bound dimers of nitrile bases has been studied using mass-analyzed ion kinetic energy spectra in a reverse geometry double focusing instrument coupled to a high pressure ion source. A linear correlation was found between the relative ion abundance ratios of the fragments and the ion source temperature. Proton exchange equilibria between the bases were studied using the time resolved capability of the high pressure mass spectrometry system, and relative ΔG°, ΔH°, and ΔS° values were extracted from the van't Hoff plots. The proton affinities (PA) of the nitriles studied were found to be PA(MeCN) = 186.8 kcal/mol, PA(EtCN) = 189.9 kcal/mol, PA (n-PrCN) = 191.0 kcal/mol, and PA(i-PrCN) = 192.2 kcal/mol. The effective temperatures of the metastable protonated nitrile dimers at different ion source temperatures were obtained from a plot of In(IRCN/Iref.) versus ΔGB (relative gas phase basicity). From a plot of the effective temperature of the protonated metastable nitrile dimers reacting in the second field free region of the mass spectrometer versus the ion source temperature, it was found that the effective temperature decreases with increasing ion source temperature, which can be explained from qualitative considerations of the relevant thermal Boltzmann distributions.
International Journal of Mass Spectrometry. 176:87-97.
