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Gas-phase clusters of protonated methylamine and phenylalanine (Phe) derivatives have been studied using infrared multiple photon dissociation (IRMPD) spectroscopy in combination with electronic structure calculations at the MP2/aug-cc-pVTZ//B3LYP/6-311+G(d,p) level of theory. Experiments were performed on several Phe derivatives including 4-chloro-l-phenylalanine (4Chloro-Phe), 4-nitro-l-phenylalanine (4Nitro-Phe), 3-cyano-l-phenylalanine (3Cyano-Phe), and 3-trifluoromethyl-l-phenylalanine (3CF3-Phe). Through comparisons between experimental IRMPD spectra and stimulated spectra obtained by electronic structure calculations, charge-solvated structures were found to be prevalent in both 4Chloro-Phe and 4Nitro-Phe, whereas 3Cyano-Phe favored zwitterionic structures and 3-CF3-Phe likely have both zwitterionic and charge-solvated structures present.

The binding motifs in the halide adducts with tyrosine ([Tyr + X]−, X = Cl, Br, I) have been investigated and compared with the analogues with 3-nitrotyrosine (nitroTyr), a biomarker of protein nitration, in a solvent-free environment by mass-selected infrared multiple photon dissociation (IRMPD) spectroscopy over two IR frequency ranges, namely 950–1950 and 2800–3700 cm−1. Extensive quantum chemical calculations at B3LYP, B3LYP-D3 and MP2 levels of theory have been performed using the 6-311++G(d,p) basis set to determine the geometry, relative energy and vibrational properties of likely isomers and interpret the measured spectra. A diagnostic carbonyl stretching band at ∼1720 cm−1 from the intact carboxylic group characterizes the IRMPD spectra of both [Tyr + X]− and [nitroTyr + X]−, revealing that the canonical isomers (maintaining intact amino and carboxylic functions) are the prevalent structures. The spectroscopic evidence reveals the presence of multiple non-covalent forms. The halide complexes of tyrosine conform to a mixture of plane and phenol isomers. The contribution of phenol-bound isomers is sensitive to anion size, increasing from chloride to iodide, consistent with the decreasing basicity of the halide, with relative amounts depending on the relative energies of the respective structures. The stability of the most favorable phenol isomer with respect to the reference plane geometry is in fact 1.3, −2.1, −6.8 kJ mol−1, for X = Cl, Br, I, respectively. The change in π-acidity by ring nitration also stabilizes anion–π interactions yielding ring isomers for [nitroTyr + X]−, where the anion is placed above the face of the aromatic ring.

A Hammett plot is a useful tool in physical organic chemistry for determining the effect of substituents on reaction rates and equilibria. In this work, gas phase energetics calculated at the relatively low computational cost B3LYP/6-311++G** level of theory for the ionization of benzoic acids and phenols have been shown to be in excellent agreement with experimental values. These data were used to generate theoretical Hammett plots for the gas phase ionization of substituted phenylacetic acids and phenols, in which good correlation was shown. To determine the effect of solvation on ionization, the calculated energetics for the gas phase ionization of benzoic acids and phenols were compared to experimental solution phase energetics. Resulting ρ values obtained demonstrated the pronounced effect of solvent in damping substituent effects. Overall, a good correlation between experimental solution phase and computational gas phase energetics was shown.

The thermochemistry of gas-phase ion-molecule interactions and structures of a variety of clusters formed between protonated amino acids and either ammonia or amines have been studied by pulsed ionization high-pressure mass spectrometry (HPMS) and ab initio calculations. The enthalpy changes for the association reactions of protonated Gly, Ala, Val, Leu, Ile, Ser, and Pro with ammonia have been measured as -23.2, -21.9, -21.0, -20.8, -20.6, -22.6, and -20.4 kcal mol(-1), respectively. A very good linear relationship exists between the enthalpy changes and the proton affinities (PAs) of the amino acids, with an exception of Ser, where the hydroxyl substituent forms an extra hydrogen bond with ammonia. For the association reaction of protonated proline and methylamine, the measured enthalpy and entropy changes are -26.6 kcal mol(-1) and -30.1 cal mol(-1) K(-1), respectively. The experimental and calculated results indicate that the zwitterionic structure of proline may be well stabilized by CH3NH3(+). For the first time, the interaction strengths between these amino acids and NH4(+) have been obtained, and comparison with Na+ is discussed. Stabilization of zwitterionic structures of a series of amino acids (Gly, Ala, Val, Ser, and Pro) by various ammonium ions (NH4(+), CH3NH3(+), (CH3)2NH2(+), and (CH3)3NH+) has been investigated systematically. Energy decomposition analysis has been performed so that the salt bridge interaction strengths between zwitterionic amino acids and ammonium ions have been obtained. Some generalizations with respect to the relative stability of zwitterionic structures may be drawn. First, as the PA of an amino acid increases, within a series of Gly, Ala, Val, the zwitterionic structure becomes more energetically favorable relative to a non-zwitterionic isomer. Second, as the PA of an amine increases, the zwitterionic structure of a given amino acid within the complex becomes gradually less favorable. Third, compared to the other amino acids, Pro, the only secondary amine among the 20 naturally occurring amino acids, has a much more pronounced tendency to form the zwitterionic structure, which has been confirmed by the experimental results. Finally, substituents on the amino acid backbone that may participate in additional hydrogen bond interactions in non-zwitterionic isomer may render it more stable, as seen in Ser. These organic ammonium ions are found to be able to very effectively stabilize the zwitterionic structure of amino acids, even more effectively than metal ions, which aids significantly in the understanding of why zwitterionic structures exist extensively in biological systems.

The formation of strong and very strong hydrogen bonds between NFnH3−n (n = 0.2) and F− have been computationally investigated via B3LYP/6-311++G(d,p) geometry optimizations and MP2/6-311++G(d,p) single point energy calculations. The enthalpy of deprotonation of NF2H was found to be 1518.5 kJ mol−1. The hydrogen bond energy in NH3⋯F−, NFH2⋯F− and NF2H⋯F− were calculated to be 67.9, 120.2 and 181.2 kJ mol−1, respectively, and clearly show the effect of fluorination on hydrogen bond strength in amine–fluoride systems. The change in enthalpy and entropy for the clustering of methanol to NF2H⋯F− to form the fluoride bound dimer of methanol and difluoramine has been measured via high pressure mass spectrometry to be 68.3 kJ mol−1 and 90.5 J K−1 mol−1. These values are in excellent agreement with calculated values of 70.9 kJ mol−1 and 88.5 J K−1 mol−1.

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.

A combination of infrared multiple photon dissociation (IRMPD) spectroscopy and density functional theory calculations have been employed to study the structures and mode-specific dissociation pathways of the proton-bound dimer of 3-trifluoromethylphenylalanine (3-CF3-Phe) and trimethylamine (TMA). Three structural motifs are identified: canonical (charge-solvated), zwitterionic (charge-separated), and TMA-bridged. In the 1000-1350 cm(-1) region, similar spectra are observed in the TMA·H(+) and 3-CF3-Phe·H(+) product channels. At wavenumbers above 1350 cm(-1), infrared excitation of charge-solvated structures leads exclusively to production of protonated TMA, while excitation of zwitterionic or TMA-bridged structures results exclusively in production of protonated 3-CF3-Phe. The cluster potential energy landscape is topologically mapped and mechanisms for isomerization and mode-selective dissociation are proposed. In particular, cluster transparency as a result of IR-induced isomerization is implicated in deactivation of some IRMPD channels.

The equilibria involved in the formation of the proton-bound dimers of the three simplest amino acid methyl esters (glycine, alanine and valine methyl esters) were characterized by means of pulsed ionization high pressure mass spectrometry (PHPMS) experiments and density functional theory (DFT) calculations. Our results would indicate that, within the temperature range employed in these experiments, the most stable proton-bound dimer conformers are formed in the case of glycine and alanine whereas a more entropically favoured isomer would dominate in the case of valine. Various possible isomers of each of the proton-bound dimer species have been investigated computationally each exhibiting a combination of inter- and intramolecular hydrogen bonding. A system of nomenclature for these various species is proposed. The possibility of structures exhibiting 'salt-bridge’ interactions have also been explored, recognizing that such structures would necessarily result from highly energetic structural rearrangements.

Mass spectrometry-based methods have been employed in order to study the reactions of non- (h6/h6), half (d6/h6), and fully (d6/d6) 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 vsm/z 59) from the dissociation reaction of half deuterium labeled (d6/h6) protonated dimer of acetone is dependent on the internal energy. A relative ion yield (m/z 65 vsm/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. Copyright © 2005 John Wiley & Sons, Ltd.

The stationary points on the potential energy surfaces for a number of gas-phase S(N)2 reactions have been determined using a combination of pulsed ionization high-pressure mass spectrometry. MP2/6-311++G**//B3LYP/6-311+G** calculations are shown to provide excellent agreement with the experimentally determined values, providing confidence for the use of this computational method to predict values that are not available experimentally. The binding in the halide/nitrile complexes has been described in the past as either hydrogen bonding or electrostatic bonding. The trends in the binding energies observed here, though, cannot be rationalized in terms of simply hydrogen bonding or ion-dipole bonding but a mixture of the two. The computed structures support the description of binding as a mixture of hydrogen bonding and ion-dipole bonding.

Benzylium versus tropylium ion yields from the fragmentation of ethylbenzene cations at various excitation energies are studied by forming excited ethylbenzene cations by charge transfer from a series of chargetransfer agents and by identifying the benzylium ion by its secondary reaction with neutral ethylbenzene. At lower excitation energies, the tropylium ion yield decreases with increasing energy from values near 16% (at an energy of 230 kJ mol -1 ) to 5% (at an energy of 500 kJ mol -1 ). At higher excitation energies, the tropylium ion yield increases again, which is attributed to secondary isomerization of the vibrationally highly excited benzylium ion arising from the primary fragmentation. It is suggested that this isomerization competes with radiative cooling of the excited benzylium ion. The experimental observations are rationalized in the framework of statistical unimolecular rate theory and electronic structure calculations.

Alkyl cation transfer reactions between ethanol and protonated ethanol and ethanol and protonated propionitrile, as well as between n-propanol and protonated n-propanol have been investigated experimentally by low-pressure Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. The two ethyl cation transfer reactions were found to be the dominant reaction channels with association being only a minor reaction pathway. The n-propyl cation transfer reaction was found to compete with the association pathway resulting in an approximately 50:50 mixture of protonated di-n-propyl ether and the proton-bound dimer of n-propanol, depending on temperature and pressure. The enthalpies of activation were determined to be −16.1 ± 0.8, −17.5 ± 0.8, and −15.7 ± 0.9 kJ mol-1 for the ethanol/protonated ethanol, ethanol/protonated propionitrile, and n-propanol/protonated n-propanol alkyl cation transfer reactions, respectively. The entropies of activation were found to be essentially the same, −121 ± 28 J K-1 m...

The reactions of protonated diethyl ether with ethanol and protonated di-n-propyl ether with n-propanol, producing the mixed proton-bound dimers, were studied at low pressures in a FTICR cell. The pressure dependence of the apparent rate constant for proton-bound dimer formation was investigated and yielded unimolecular dissociation rate constants, kb, and photon emission rate constants, kra, for the nascent proton-bound dimers at internal energies equal to the dissociation energies of the dimers. The experimental kra values were found to be 17 ± 3 and 6.3 ± 0.6 s-1, respectively, for the ethanol/diethyl ether and n-propanol/di-n-propyl ether proton-bound dimers. RRKM modeling of the unimolecular dissociation rate constants as a function of the binding energies yielded the 0 K dissociation energies of the proton-bound dimers as 109 ± 1 and 105.1 ± 0.6 kJ mol-1 for the ethanol/diethyl ether and n-propanol/di-n-propyl ether proton-bound dimers, respectively. Using B3LYP/6-311G** thermal energies, the corres...

The enthalpy and entropy changes for the formation of the 1:1 complexes of methanol with various gaseous protonated amino acids have been measured using pulsed-ionization high-pressure mass spectrometry. The enthalpy changes for formation of the clusters Gly(MeOH)H(+), Ala(MeOH)H(+), Val(MeOH)H(+), Leu(MeOH)H(+), Ile(MeOH)H(+), and Pro(MeOH)H(+) have been determined to be -92.0, -83.3, -82.4, -79.5, -78.7, and -73.6 kJ mol(-1), respectively. These values agree very well with the energetic values computed at the MP2(full)/6-311++G(2d,2p)//B3LYP/6-311+G(d,p) level of theory for the lowest energy adducts in each system. Both experimental observations and computational determinations of the potential energy surface for the glycine system suggest that a mixture of low-lying isomers may be present for each of the cluster systems examined. The primary structural motif for these clusters is the coordination of the methanol molecule to the ammonium group of the protonated amino acid via a strong ionic hydrogen bond. For the amino acids studied here, computational results reveal that one methanol molecule does not sufficiently stabilize any zwitterionic structure such that no appreciable extent of proton transfer from the amino acid to methanol was observed.

C–H···X (where X = O or S) intramolecular hydrogen bonding is investigated in three benzyl Meldrum’s acid derivatives using a combination of solution phase NMR spectroscopy, gas phase infrared multiple photon dissociation spectroscopy, and density functional theory calculations. In one compound, an abnormally large C–H···S hydrogen bond energy of 30.4 kJ mol–1 is calculated with a natural bond orbital analysis. Intramolecular C–H···O hydrogen bonding is found to persist in the gas phase. Gibbs energy decomposition pathways are calculated.

The thermochemistry of the chloride ion clustering onto dimethyl ether, diethyl ether, and three fluorinated ethers (CH 3 OCF 3 , (CF 2 H) 2 O, and CF 3 OCF 2 H) under thermal equilibrium conditions has been determined using pulsed-ionization high pressure mass spectrometry. The standard enthalpy (Δ H °) and entropy change (Δ S °) 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. Δ H ° 298 value calculated at the MP2/[6-311 ++ G(3 df ,3 pd )/6-311 + G(2 df , p )]//MP2/[6-31 + G( d )/6-31G( d )] level of theory show excellent agreement with the experimentally obtained Δ H ° values.

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.

Methyl cation transfer reactions between methanol and protonated methanol, protonated acetonitrile, and protonated acetaldehyde have been investigated experimentally by low-pressure FT-ICR mass spectrometry. The temperature dependencies of the rate constants for these reactions were determined in an Arrhenius-type analysis to obtain activation energies, enthalpies, and entropies of activation. The enthalpies of activation were determined to be -16.9 ( 0.6, -16.5 ( 0.6, and -18.4 ( 0.7 kJ mol -1 for the methanol/protonated methanol, methanol/protonated acetonitrile, and methanol/protonated acetaldehyde reactions, respectively. These values agree quite well with ab initio-calculated values. The entropies of activation were found to be quite similar for all three reactions within experimental uncertainty, which is expected due to the similar transitionstate structures for all reactions. Ab initio potential energy surfaces calculated at the MP2/6-311G** level and basis set are reported for the three reactions. For the methanol/protonated acetonitrile and methanol/ protonated acetaldehyde reactions, isomerization of the initially produced proton-bound dimer to a methylbound complex is suggested prior to methyl cation transfer. The barrier for the first isomerization is predicted to be significantly lower than the barrier for methyl cation transfer such that it does not interfere with the experimental determination of the latter.

The temperature dependence of the low-pressure association reaction of dimethyl ether with protonated dimethyl ether has been investigated using Fourier transform ion cyclotron resonance mass spectrometry. The unimolecular dissociation of nascent proton-bound dimers is complicated by two factors: (1) the presence of another unimolecular dissociation route producing trimethyloxonium cation and methanol through a highenergy isomer of the proton-bound dimer and (2) the presence of at least two high-energy isomers of the proton-bound dimer en route to dissociation of the nascent proton-bound dimer. RRKM modeling of the experimental temperature dependence of the unimolecular dissociation of nascent proton-bound dimers strongly suggests that dissociation of the nascent proton-bound dimer proceeds through a high-energy isomer. The possible existence of such species is also shown by ab initio calculations. The original mechanism for the ion/molecule reaction and analysis of radiative association kinetics used in the past was found to be too simple for accurate modeling of the reaction between protonated dimethyl ether and dimethyl ether. A slightly more complicated mechanism is proposed which more accurately accounts for the temperature dependence of the unimolecular dissociation to re-form reactants. As well, three isotopomeric variants of the protonated dimethyl ether/dimethyl ether reaction were examined experimentally and theoretically.

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) lev...

Fourier transform ion cyclotron resonance (FTICR) measurements of solvent exchange equilibria, where the solvents differ only by a single deuterium substitution at the labile hydroxylic site, have been used to study the deuterium isotope effect on ion–molecule hydrogen-bonding interactions. The systems studied include chloride ion, fluoride ion, and alkoxide ions solvated by up to three molecules of alcohol: (ROH)n · Cl−, (ROH)n · F−, and (ROH)n · RO−. Differences in the measured isotope effects are explained on the basis of the differing hydrogen bond strengths in the adducts and by the ability of the chloride ion to partake in multiple site interactions. Keywords: Kinetic isotope effect; Hydrogen bonding; Fourier transform ion cyclotron resonance (FTICR); Ion–molecule reactions

Fourier transform ion cyclotron resonance experiments have been carried out to probe the nature of the potential energy surface for the reaction of methyl cation (CH3+) with methane (CH4) to give the ethyl cation (C2H5+). Product distributions for reactions of CH3+ and CD3+ with CD4 and CH4, respectively, are found to give a near statistical distribution of ethyl cation products, in good agreement with previous work. When the methyl cation is initially coordinated to HF in the form of a methylfluoronium ion, however, the product distributions are decidedly nonstatistical and are indicative of a reaction which is very nearly thermoneutral. Thermochemical data for relevant species suggest that the reaction is very slightly endothermic. All of the experiments support the intermediacy of a C2H7+ complex in the reactions. Ab initio calculations, in conjunction with all of the experimental data, reveal that there are likely three different stable forms of C2H7+ involved in the potential energy surface for the reaction. One of the forms, a nonclassical C2H5+ coordinated to H2 in a proton bound dimerlike structure, has not previously been considered to play a role in this reaction. The existence of this structure is supported by infrared multiphoton dissociation experiments on C2H7+ previously carried out by Lee and co-workers (J. Am. Chem. Soc. 111 (1989) 5597) and the high pressure mass spectrometric experiments of Hiraoka and Kebarle (J. Am. Chem. Soc. 98 (1976) 6119).

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.

Kinetic and thermodynamic isotope effects on the unimolecular dissociation of proton bound dimers were studied in the gas phase using mass spectrometry techniques. In addition proton transfer reactions were investigated using equilibrium techniques in conjunction with a theoretical study. Normal isotope effects were observed for all of the amine systems studied. The effect of label position, extent of labeling, size and structure of the proton bound dimers have been discussed with respect to (i) the kinetic and thermodynamic isotope effect on the dissociation reaction, (ii) the kinetic energy release on the dissociation reaction, (iii) the thermodynamic isotope effect on the proton exchange reaction between the labeled and unlabeled amines, and (iv) the effective temperatures and the excess energies of the metastable proton bound dimers. Other compound classes (CH3OH, (CH3)2O, CH3CN and (CH3)2CO) were studied and discussed in the same way, though not as thoroughly. All the systems show normal isotope effects, except for the proton bound dimer of CH3CN and CD3CN, which showed an inverse isotope effect.

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(I RCN /I ref. ) 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.

Fourier Transform Ion Cyclotron Resonance (FT-ICR) and High Pressure Mass Spectrometric (HPMS) measurements of the deuterium isotope effect and kinetics of adduct ion formation have been used to probe the nature of the potential describing the motion of the hydrogen in gas phase ion-molecule hydrogen-bonding interactions. Hydrogen-bonding systems reported in this paper are alkoxide ion and chloride ion solvated by one molecule of alcohol, ROH●OR- and ROH●Cl-. Significant differences in the isotope effects were observed for the two systems. These differences are explained on the basis of the differing hydrogen bond strengths of the adduct ions, and the ability of the chloride ion to partake in multiple site, or chelate, interactions. In addition, HPMS studies of the kinetics of the reaction of CH3O− with CH3OH reveal that a double minimum potential energy surface may be appropriate for describing the adduct ion formation. These experimental studies have been supplemented by ab initio calculations to determine adduct ion structures as well as to permit statistical thermodynamic calculations of the isotope effect.

As a result of an extensive delocalization of charge and a unique covalent structure, the CH3OCH2+ cation has, in effect, the character of an ambident electrophile. This cation can, on the one hand, be considered to be a classical electrophile. On the other hand, it may be considered to be a facile methyl cation donor. The former character predominates when this cation reacts with alcohols, as is shown in this work. In both the chemical ionization (CI) source of a conventional mass spectrometer and also via low-pressure bimolecular reactions in a Fourier transform ion cyclotron resonance (ICR) cell, the dominant reaction between alkoxymethyl cations and alcohols is the very exothermic formation of a C—O bond to give a covalent adduct having the structure of a protonated dialkoxymethane. A 1,3-hydrogen transfer is observed for the covalent adducts. In the case of those generated in the ICR cell this process is a slow unimolecular reaction. However, the rapid 1,3-hydrogen transfer observed in the CI source is a bimolecular reaction catalysed by a second molecule of alcohol. This is a new example of catalysed isomerization in the gas phase. In competition with the 1,3-hydrogen transfer, the covalent adducts may either undergo simple bond cleavage or may isomerize to proton-bound dimer adducts of ether and aldehyde (or ketone) via a hydride transfer mechanism. This mechanism either may involve an electrostatic complex intermediate or may be an asynchronous concerted process. Since the proton affinities of the ethers involved in these proton-bound dimer intermediates are greater than those of the aldehydes derived from primary alcohols, such dimers dissociate to yield protonated ether and aldehyde. Conversely, those dimers resulting from secondary alcohols involve ketones whose proton affinities are greater than those of the partner ethers and these dimers dissociate to yield protonated ketone and ether. In summary, the reactions of CH3OCH2+ with alcohols occur via several successive and specific steps. © 1997 by John Wiley & Sons, Ltd.

The gas-phase reactivity of W+ with hydrocarbons containing up to six carbon atoms has been studied by Fourier-transform ion cyclotron resonance mass spectrometry. W+ is found to be one of the most reactive bare metal cations studied to date with all hydrocarbons except acetylene being activated at thermal energies. The observed reactivity is dominated by dehydrogenation, which is often multiple. Thus, reaction with propene leads to loss of two dihydrogen molecules (yielding WC3H2) and reaction with n-hexane leads to loss of 4 H2 (yielding WC6H6). In all cases, the product ions are themselves reactive with the same neutral species, leading to reaction sequences of various lengths. Isotope labeling, collisioninduced dissociation, and ligand exchange experiments have been extensively carried out in order to try to establish the structure of some of the intermediate and final product ions. It turned out that deriving structures from such experiments is much more difficult than for first-row transition metal ions. This is due to the higher reactivity and metal-ligand binding energies of tungsten.

Infrared multiple photon dissociation (IRMPD) spectroscopy experiments and quantum chemical calculations have been used to explore the possible structures of protonated azidothymidine and the corresponding protonated dimer. Many interesting differences between the protonated and neutral forms of azidothymidine were found, particularly associated with keto-enol tautomerization. Comparison of computational vibrational and the experimental IMRPD spectra show good agreement and give confidence that the dominant protonated species has been identified. The protonated dimer of azidothymidine exhibits three intramolecular hydrogen bonds. The IRMPD spectrum of the protonated dimer is consistent with the spectrum of the most stable computational structure. This work brings to light interesting keto-enol tautomerization and exocyclic hydrogen bonding involving azidothymidine and its protonated dimer. The fact that one dominant protonated species is observed in the gas phase, despite both the keto and enol structures being similar in energy, is proposed to be the direct result of the electrospray ionization process in which the dominant protonated dimer structure dissociates in the most energetically favorable way.

The gas-phase reactions of the bare tungsten cation W+ with silane have been investigated using Fourier-Transform Ion Cyclotron Resonance mass spectrometry. Dehydrogenation of a first molecule leads to the formation of WSiH2+. This ion is itself reactive with a second silane molecule, this time through elimination of 2H2, to form WSi2H2+. A similar reaction follows, yielding WSi3H2+ as the next product ion, which itself leads to both WSi4H4+ and WSi4H2+. This seems to initiate two parallel reaction sequences, yielding WSi10H6+ as the major final product, together with a minor amount of WSi10H4+. CID experiments on the products of the first three reactions were carried out to aid in their structural elucidation. Ab initio calculations at the CASSCF level have been performed in order to derive optimum structures for the first two product ions WSiH2+ and WSi2H2+, and for the non-observed intermediate WSi2H4+. The results show that structural isomerism exists for these three ions, due to the versatile bonding...

Protonated ferulic acid and its principle fragment ion have been characterized using infrared multiple photon dissociation spectroscopy and electronic structure calculations at the B3LYP/6-311 + G(d,p) level of theory. Due to its extensively conjugated structure, protonated ferulic acid is observed to yield three stable fragment ions in IRMPD experiments. It is proposed that two parallel fragmentation pathways of protonated ferulic acid are being observed. The first pathway involves proton transfer, resulting in the loss of water and subsequently carbon monoxide, producing fragment ions m/z 177 and 149, respectively. Optimization of m/z 177 yields a species containing an acylium group, which is supported by a diagnostic peak in the IRMPD spectrum at 2168 cm(-1). The second pathway involves an alternate proton transfer leading to loss of methanol and rearrangement to a five-membered ring.

Ionic hydrogen-bonding interactions have been found in several clusters formed by 5-fluorocytosine (5-FC). The chloride and trimethylammonium cluster ions, along with the cationic (proton-bound) dimer have been characterized by infrared multiple-photon dissociation (IRMPD) spectroscopy and electronic structure calculations performed at the B2PLYP/aug-cc-pVTZ//B3LYP/6-311+G(d,p) level of theory. IRMPD action spectra, in combination with calculated spectra and relative energetics, indicate that it is most probable that predominantly a single isomer exists in each experiment. For the 5-FC-trimethylammonium cluster specifically, the calculated spectrum of the lowest-energy isomer convincingly matches the experimental spectrum. Interestingly, the cationic dimer of 5-FC was found to have a single energetically relevant isomer (Cationic-IV) involving a tridentate ionic hydrogen-bonding interaction. The three sites of intermolecular ionic hydrogen bonds in this isomer interact very efficiently, leading to a significant calculated binding energy of 180 kJ/mol. The magnitude of the calculated binding energy for this species, in combination with the strong correlation between the simulated and IRMPD spectra, suggests that a tridentate-proton-bound dimer was observed predominantly in the experiments. Comparison of the calculated relative Gibbs free energies (298 K) for this species and several of the other isomers considered also supports the likelihood of the dominant protonated dimer existing as Cationic-IV.

The association reactions of methylated acetone with acetone and dimethyl ether were studied over an extended temperature range (30–250°C) by high-pressure mass spectrometry. In addition, the association reactions of various deuterium-substituted analogues of both the reactant ion and neutral molecules were investigated. Two distinct isomeric forms are apparent from the examination of the behaviour of the association equilibrium constant as a function of reaction temperature: (i) a low-temperature, covalently bound isomer characterized by larger −ΔH° and −ΔS° values and significant equilibrium isotope effects on the entropy of association; and (ii) a high-temperature, electrostatically bound isomer of lower −ΔH° and −ΔS° values and with insignificant equilibrium isotope effects. The equilibrium isotope effects and larger −ΔS° values are more pronounced for the methylated acetone–dimethyl ether adduct than for the less sterically congested methylated acetone-acetone adduct. Molecular models of the low-temperature isomers show severe restriction or complete loss of internal rotations and is the origin of the observed equilibrium isotope effects. No such phenomenon is expected or observed in the more loosely bound high-temperature, electrostatic isomers. Potential energy surfaces for the transfer of methylcation from methylated acetone to a second acetone molecule and to dimethyl ether are proposed.

The structure of the proton-bound lysine dimer has been investigated by infrared multiple photon dissociation (IRMPD) spectroscopy and electronic structure calculations. The structures of different possible isomers of the proton-bound lysine dimer have been optimized at the B3LYP/6-31 + G(d) level of theory and IR spectra calculated using the same computational method. Based on relative Gibbs free energies (298 K) calculated at the MP2/aug-cc-pVTZ//B3LYP/6-31 + G(d) level of theory, LL-CS01, and followed closely (1.1 kJ mol(-1)) by LL-CS02 are the most stable non-zwitterionic isomers. At the MP2/aug-cc-pVTZ//6-31 + G(d) and MP2/aug-cc-pVTZ//6-31 + (d,p) levels of theory, isomer LL-CS02 is favored by 3.0 and 2.3 kJ mol(-1), respectively. The relative Gibbs free energies calculated by the aforementioned levels of theory for LL-CS01 and LL-CS02 are very close and strongly suggest that diagnostic vibrational signatures found in the IRMPD spectrum of the proton-bound dimer of lysine can be attributed to the existence of both isomers. LL-ZW01 is the most stable zwitterionic isomer, in which the zwitterionic structure of the neutral lysine is well stabilized by the protonated lysine moiety via a very strong intermolecular hydrogen bond. At the MP2/aug-cc-pVTZ//B3LYP/6-31 + G(d), MP2/aug-cc-pVTZ//6-31 + G(d) and MP2/aug-cc-pVTZ//6-31 + G(d,p) levels of theory, the most stable zwitterionic isomer (LL-ZW01) is less favored than LL-CS01 by 7.3, 4.1 and 2.3 kJ mol(-1), respectively. The experimental IRMPD spectrum also confirms that the proton-bound dimer of lysine largely exists as charge-solvated isomers. Investigation of zwitterionic and charge-solvated species of amino acids in the gas phase will aid in a further understanding of structure, property, and function of biological molecules.

Fourier transform ion cyclotron resonance measurements of the deuterium isotope effect have been used to probe the nature of the potential describing the motion of the hydrogen in gas phase ion/molecule hydrogen bond interactions. The hydrogen bonding system studied is fluoride ion solvated by one molecule of aliphatic alcohol, ROH · F−. No variation of the isotope effect with alcohol acidity was observed for unsubstituted aliphatic alcohols. Arguments are presented which imply that the internal rotation of the neutral alcohols plays an important role in the determination of the overall isotope effect, as well as the more commonly considered zero-point energy effects.

An examination of strong hydrogen bonds found in (ROH)(n)...FHF(-) clusters (n = 1 and 2; R = H, CH(3), C(2)H(5)) is presented. Excellent agreement is observed between thermochemical values obtained from high-pressure mass spectrometric measurements and those predicted from MP2(full)/6-311++G(d,p)//B3LYP/6-311++G(d,p) calculations. Calculated structures are examined, and insight into the geometric nature of the bonding for these systems is obtained. In the case of water binding to FHF(-), it was found that the large entropic advantage of one particular structure, which was not the most enthalpically favored, was significant enough to make it the predominant species within the ion source. In the case of methanol solvation, no evidence of secondary interaction of the methyl group and any other moiety could be found. The structural details revealed from calculations of the ethanol-solvated clusters indicate that secondary interactions between the terminal methyl group and FHF(-) have an impact on the length of the FHF and OHF bonds.

The formation of C6H7+ species in ion/molecule reactions in gaseous vinyl chloride was studied in a high pressure photoionization mass spectrometer and in a Fourier transform ion cyclotron resonance (FT-ICR) spectrometer. Collision-induced dissociation (CID) mass spectra of C4H5Cl+, C4H6Cl+, and C6H7+ species suggest a "butadiene-like" structure for the two former ions, and a non-benzenium structure for the last species. The C6H7+ ions are formed in a two-step mechanism involving C4H5+ as intermediate ions. These processes are in competition with condensation reactions leading to the formation of C6H7–9Cl+ species. Key words: ion–molecule reactions, gaseous vinyl chloride, collision-induced dissociation.

The thermochemical properties associated with the formation of an isomeric distribution of ROHNH(2)CH(2)COO(-) clusters (R=H, CH(3), C(2)H(5)) are measured by using high-pressure mass spectrometry. A comparison of the measured properties with calculated values provides new insights into the thermochemical effects arising from the isomeric nature of this clustering system. When the distribution of isomers is correctly accounted for, the measured values of DeltaH degrees , DeltaS degrees , and DeltaG degrees (298) consistently agree, to a very high degree of accuracy, with those predicted by MP2(full)/6-311++G(d,p)//B3LYP/6-311++G(d,p) calculations.

Zwitterionic structures exist extensively in biological systems and the electric field resulting from zwitterion formation is the driving force for determination of the properties, function and activity of biological molecules, such as amino acids, peptides and proteins. It is of considerable interest and import to investigate the stabilization of zwitterionic structures in the gas phase. Infrared multiple photon dissociation (IRMPD) spectroscopy is a very powerful and sensitive technique, which may elucidate clearly the structures of both ions and ionic clusters in the gas phase, since it provides IR vibrational fingerprint information. The structures of the clusters of glycine and ammonium ion and of the betaine proton-bound homodimer have been investigated using IRMPD spectroscopy, in combination with electronic structure calculations. The experimental and calculated results indicate that zwitterionic structure of glycine may be effectively stabilized by an ammonium ion. This is the smallest zwitterionic structure of an amino acid to be demonstrated in the gas phase. On the basis of the experimental IRMPD and calculated results, it is very clear that a zwitterionic structure exists in the proton-bound betaine dimer. The proton is bound to one of the carboxylate oxygens of betaine, rather than being equally shared. Investigations of zwitterionic structures in the isolated state are essential for an understanding of the intrinsic characteristics of zwitterions and salt bridge interactions in biological systems.

The low pressure association reactions for the proton bound dimers of acetonitrile, acetone, dimethyl ether and diethyl ether have been studied using Fourier transform-ion cyclotron resonance spectrometry. Values for the rate constants for the unimolecular decomposition and radiative stabilization reactions of the intermediate proton bound dimer are determined. Trends of molecular size and number of normal modes are examined for unimolecular lifetimes of the association intermediate. No apparent trends are observed to allow the prediction of radiative stabilization lifetimes.

Protonation sites in biological molecules are very important because they directly determine their structures, properties, and functions. The protonation sites and structures of the tripeptides of Gly and Ala have been investigated using infrared multiple photon dissociation (IRMPD) spectroscopy in combination with theoretical calculations. The experimental and calculated results indicate that two isomers coexist under the experimental conditions. Different protonation sites result in the different structures. If the excess proton is bound to the amino group, a cyclic structure is formed, in which the N-terminus and carbonyl oxygen of the C-terminus are linked by a strong hydrogen bond. In contrast, if the excess proton is bound to the amide carbonyl oxygen, a linear structure is formed in which a very strong hydrogen bond is formed, linking the two amide carbonyls. For the first time, IR spectroscopic evidence confirms that an amide oxygen may serve as the protonation site for peptides.

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.

International audience; Infrared multiphoton dissociation spectra of three homogeneous and two heterogeneous proton-bound dimers were recorded in the gas phase. Comparison of the experimental infrared spectra recorded in the fingerprint region of the proton-bound dimers with spectra predicted by electronic structure calculations shows that all modes which are observed contain motion of the proton oscillating between the two monomers. The O-H-O asymmetric stretch for the homogeneous dimers is shown to occur at around 800 cm-1. As expected, the O-H-O asymmetric stretching modes for the heterogeneous proton-bound dimers are observed to shift to significantly higher energy with respect to those for the homogeneous proton-bound dimers due to the asymmetry of the O-H-O moeity. This shift is shown to be predictable from the difference in proton affinities between the two monomers. Density functional predictions of the infrared spectra based on the harmonic oscillator model are demonstrated to predict the observed spectra of the homogeneous proton-bound dimers with reasonable accuracy. Calculations of the structure and infrared spectrum of protonated diglyme at the B3LYP/6-31+G** level and basis also agree well with an infrared spectrum recorded previously. For both heterogeneous proton-bound dimers, however, the predicted spectra are blue-shifted with respect to experiment.

International audience; In the gas phase, the CH2CHOH.+ enol radical cation 1 as well as its higher homologues CH3CHCHOH.+ 2 and (CH3)2CCHOH.+ 3, undergo exactly the same sequence of reactions with tert-butanol, leading to the losses of isobutene, water and water plus alkene. Fourier transform ion cyclotron resonance (FT-ICR) experiments using labeled reactants as well as ab initio calculations show that independent pathways can be proposed to explain the observed reactivity. For ion 1, taken as the simplest model, the first step of the reaction is formation of a proton bound complex which gives, by a simple exothermic proton transfer, the ter-body intermediate [CH2CHO., H2O, C(CH3)3+]. This complex, which was shown to possess a significant lifetime, is the key intermediate which undergoes three reactions. First, it can collapse to yield tert-butylvinyl ether with elimination of water. Second, by a regiospecific proton transfer, this complex can isomerize into three different ter-body complexes formed of water, isobutene and ionized enol. Within one of these complexes, which does not interconvert with the others, elimination of isobutene leads to the formation of a solvated enol ion. Within the others, a cycloaddition-cycloreversion process can proceed to yield the ionized enol 3 (loss of water and ethylene channel).

The unimolecular isomerization of CH3COCH3+· 1 into its more stable enol counterpart CH3C(OH)CH2+· 2 is known not to occur, as a significant energy barrier separates these ions. However, it is shown in this work that this isomerization can be catalyzed within a 1 : 1 ion-neutral complex. For instance, a Fourier transform ion cyclotron resonance mass spectrometry study shows that one, and only one, molecule of isobutyronitrile catalyzes the isomerization of 1 into 2. The rather low efficiency of the reaction (12%), as well as the strong isotope effect observed when CD3COCD3+· is used as the reactant ion, suggest that the catalyzed isomerization implicates a substantial intermediate energy barrier. This was confirmed by ab initio calculations that allow us to propose an isomerization mechanism in agreement with this experiment. The efficiency of different catalysts was studied. To be efficient, the catalyst must be basic enough to abstract a proton from the methyl group of ionized acetone but not too basic to give back this proton to oxygen. In other words, the proton affinity (PA) of an efficient catalyst must lie, in a first approximation, between the PA of the radical CH3COCH2· at the carbon site (PAC) and its PA at the oxygen site (PAO), which have been determined to be, respectively, 185.5 and 195.0 kcal mol−1. Most of the neutral compounds studied follow this PA rule. The inefficiency of alcohols in the catalytic process, although their PAs lie in the right area, is discussed. Keywords: Catalyzed keto-enol tautomerism; gas-phase proton transport; isomerization kinetics; proton affinity rule; FT-ICR mass spectrometry

International audience; Fourier transform ion cyclotron resonance experiments show that a variety of molecules catalyze the hydrogen transfer which converts ionized acetaldehyde CH3CHO.+ 1 to its vinyl alcohol counterpart CH2CHOH.+ 2. Each of these ions has been characterized by its specific bimolecular reactions with selected reactants. Calculations show that two pathways, for which the rate determining barriers have almost the same energy, are feasible. The first transition state involves a direct catalyzed 1,3-H transfer, while the second involves two successive 1,2-H transfers. A detailed experimental study, using methanol as a catalyst as well as labeled reactants, indicates that only the first pathway operates in the isomerization process. The different steps of these two independent pathways were elucidated. The first begins with the formation of a highly stabilized complex 3, involving a two-center-three-electron interaction between the two oxygen atoms and an interaction between a hydrogen of the methyl group of 1 and the oxygen of methanol. This complex isomerizes into a complex 4, which in turn gives the complex 5, via a transition state located 6.3 kcal mol-1 below the energy of the reactants. This complex 5 corresponds to ionized vinyl alcohol hydrogen bonded to the oxygen of methanol, which dissociates to yield ion 2. The second pathway begins with the interaction between the hydrogen of the CHO group and the oxygen of methanol and gives the complexes 6 and then 7, which correspond to protonated methanol hydrogen bonded to a CH3CO radical. Dissociation of 7 to give protonated methanol is favoured with respect to further isomerization leading to ionized vinyl alcohol. Compared to the unimolecular conversion between energetic ions 1 and 2, which can occur either by a direct 1,3-H transfer or by a double 1,2-H transfer, the reaction of 1 with methanol catalyzes the first pathway while inhibiting the second one. In the case studied, catalysis is perhaps better described as a hydrogen atom transport. (C) 2000 Elsevier Science B.V.

Infrared multiple photon dissociation (IRMPD) spectroscopy is a very powerful technique to characterize the structures of ions and ionic clusters. The structures of Gly and Pro proton-bound homodimers have been investigated using this method in combination with theoretical calculations. For the protonated Gly dimer, a new most stable isomer, GG-CS01, has been experimentally confirmed to be the dominant species. Direct IR spectroscopic evidence indicates that zwitterionic proline exists in the protonated proline dimer in the gas phase. This zwitterionic isomer (PP-ZW01) is the dominant species, and the most stable nonzwitterionic isomer (PP-CS01) also coexists under the experimental conditions.

Ionic hydrogen bond (IHB) interactions, resulting from the association of ammonia and the two protonated methylxanthine derivatives, caffeine and theophylline, have been characterized using infrared multiphoton dissociation (IRMPD) spectroscopy and electronic structure calculations at the MP2/aug-cc-pVTZ//B3LYP/6-311+G(d,p) level of theory. The proton-bound dimer (PBD) of caffeine and ammonia exhibits a low binding energy and was found to be elusive under the experimental conditions due, most probably, to collision-induced dissociation of the complex with helium buffer gas before IRMPD irradiation. The IRMPD spectrum of a PBD of theophylline and ammonia was obtained and revealed bidentate IHB formation within the complex, which greatly increased the binding energy relative to the most stable isomer of the PBD of caffeine and ammonia. The IRMPD spectra of the protonated forms of caffeine and theophylline have also been obtained. The spectrum of protonated caffeine showed dominant protonation at the N(9) site, whereas the spectrum of protonated theophylline showed a mixture of two isomers. The first protonated isomer of theophylline exhibits protonation at the N(9) site and the second isomer demonstrated protonation at the C(6) carbonyl oxygen. The protonated carbonyl isomer of theophylline cannot be produced as a result of direct protonation and is thus suggested to be a consequence of proton-transport catalysis (PTC) initiated by the electrostatic interaction between water and N(9) protonated theophylline. Calculated anharmonic spectra have been simulated at the B3LYP/6-311+G(d,p) level of theory. It is shown that calculated anharmonic frequencies significantly outperform calculated harmonic frequencies in providing simulated IRMPD spectra in all cases.

Fourier transform ion cyclotron resonance mass spectrometry has been used to measure the reaction rates for ions derived from methylamine with dimethylamine or trimethylamine. The use of the selective ion ejection technique greatly simplifies the elucidation of the ion-molecule reaction channels. The rate constants for proton transfer from protonated metwlamine, CH3NH 3 + (m/z 32), to dimethylamine and trimethylamine are 16.1 ± 1.6 × 10−10 and 9.3 ± 0.9 × 10−10 cm3 molec−1s−1, respectively. The rate constants for charge transfer from methylamine molecular ion, CH3NH 2 + (m/z 31), to dimethylamine and trimethylamine are 9.3 ± 1.8 x 10−10 and 15.0 ± 5 × 10−10 cm3molec−1s−1, respectively.