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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.

High-pressure mass spectrometric equilibrium experiments and electronic structure calculations have been carried out to investigate the energetics of the interactions of chloride ion with a series of normal alkanes and cycloalkanes in the gas phase. The structures of the complexes obtained from the electronic structure calculations provide considerable insight into the nature of the interaction between the negatively charged ion and the alkanes, which has the character of a purely ion-induced dipole interaction. The structural information also shows how the charged species affect the confirmation of the normal alkanes.

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 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...

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...

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.

The proton affinity (PA) of a molecule in the gas phase is an expression of its fundamental basicity and is a possible factor controlling the course of many ion−molecule reactions. The formation of an intramolecular hydrogen bond increases the PA value over that of similarly sized monofunctional molecules and the values of ΔS° are an indication of an intramolecular cyclization which occurs via hydrogen bonding in protonated bifunctional molecules. The first step in exploring these thermochemical properties has been the examination of the experimental proton-transfer equilibria using both ion cyclotron resonance (ICR) and high-pressure mass spectrometric (HPMS) studies. Parallel ab initio molecular orbital (MO) calculations on the protonated species show that the cyclized structures are the most stable species, in agreement with the experimental PA and entropy observations.

A new experimental apparatus is described which permits the determination of binding energetics for metal−ligand complexes. This technique mates laser ablation for the generation of atomic metal ions with the environment of a high-pressure ion source which leads to rapid termolecular stabilization of metal ion-ligand complexes containing one or more ligands. The time resolved capability of the detection system allows equilibrium to be studied quantitatively for binding energies in the range of 5−30 kcal mol-1. Such measurements of the absolute binding energy of formaldehyde to Al+ combined with existing bimolecular Al+ exchange equilibrium data leads to an absolute Al+ affinity scale. For two ligand complexes an extensive ab initio search of the potential energy surfaces shows that the experimentally observed species involving CH3CN and (CH3)2O involve simple ligand complexation with an acute L-Al−L bond angle (L = ligand) rather than hydrogen bonded or inserted structures. In one instance a third ligand ...