Theoretical calculations and experimental values from the recent literature are used to construct and evaluate a high precision gas-phase acidity scale. Gas-phase acidities at 0 K are evaluated for 12 reference species with accurately known acidities. Using recent spectroscopic results, small but significant revisions are presented for the acidities of ammonia, water, and formaldehyde. These revised anchor acidities are applied to previous thermokinetic or equilibrium measurements of the acidities of small alkanols, ethene, and benzene. Combined with electron affinities from literature negative ion photoelectron spectroscopy measurements, the revised acidities yield the following improved bond dissociation enthalpies:  D 298(CH3O−H) = 437.7 ± 2.8 kJ/mol, D 298(C2H5O−H) = 438.1 ± 3.3 kJ/mol, D 298((CH3)2CHO−H) = 442.3 ± 2.8 kJ/mol, D 298((CH3)3CO−H) = 444.9 ± 2.8 kJ/mol, D 298(C2H3−H) = 463.0 ± 2.7 kJ/mol, and D 298(C6H5BH) = 472.2 ± 2.2 kJ/mol. Calculation of gas-phase acidities at 0 K are investigated for several levels of theory. Excellent performance at the CCSD(T)/aug-cc-pVTZ//B3LYP/aug-cc-pVTZ level is found for 16 acids composed of elements through chlorine, with a mean error of −0.2 kJ/mol and a mean absolute error of 1.5 kJ/mol.

Guided ion beam tandem mass spectrometry techniques are used to examine promotion of the symmetric bimolecular nucleophilic substitution (SN2) reaction 37Cl- + CH3 35Cl → 35Cl- + CH3 37Cl by translational energy. The translational energy threshold for this process is 45 ± 15 kJ/mol, well above the previously reported potential energy barrier height of 10−13 kJ/mol for the SN2 transition state. The collisionally activated process involves conventional SN2 back-side attack at the carbon atom, but passage over the barrier is impeded by nonstatistical dynamical constraints at collision energies just above the barrier. A significant secondary kinetic isotope effect is observed. The cross section for reaction with CH3Cl is about 20% larger than for the reaction with CD3Cl. At high energies, >410 ± 40 kJ/mol, diatomic Cl2 - products are observed. The guided ion beam apparatus and data analysis procedures are described in detail.

Energy-resolved competitive collision-induced dissociation methods are used to measure the gas-phase acidities of a series of alcohols (methanol, ethanol, 2-propanol, and 2-methyl-2-propanol). The competitive dissociation reactions of fluoride−alcohol, [F-·HOR], alkoxide−water, [RO-·HOH], and alkoxide−methanol [RO-·HOCH3] proton-bound complexes are studied using a guided ion beam tandem mass spectrometer. The reaction cross sections and product branching fractions to the two proton transfer channels are measured as a function of collision energy. The enthalpy difference between the two product channels is found by modeling the reaction cross sections near threshold using RRKM theory to account for the energy-dependent product branching ratio and kinetic shift. From the enthalpy difference, the alcohol gas-phase acidities are determined relative to the well-known values of HF and H2O. The measured gas-phase acidities are Δacid H 298(CH3OH) = 1599 ± 3 kJ/mol, Δacid H 298(CH3CH2OH) = 1586 ± 5 kJ/mol, Δacid H 298((CH3)2CHOH) = 1576 ± 4 kJ/mol, and Δacid H 298((CH3)3COH) = 1573 ± 3 kJ/mol.

An investigation of gas-phase methanol clusters (CH3OH)n, where n = 2-12, 16, and 20, was completed with a range of computational methods:  PM3, Hartree-Fock, B3LYP, MP2, and their combination using an ONIOM (our own n-layered integrated molecular orbital and molecular mechanics) method. Geometries, binding energies, and vibrational frequencies are reported. For all ab initio optimized structures, the cyclic isomer was found to be the most stable structure of all isomers investigated. The scaled OH frequency shift for n = 1-4 is found to be in good agreement with experimentally measured shifts. An ONIOM method, with the methyl group calculated at the low level and the hydroxyl group at the high level, proved to be an excellent way of reducing computational expense. The calculated enthalpies, geometries, and infrared spectra using an ONIOM method were comparable to that of a high-level calculation. Clusters were solvated using the integral equation formalism for the polarized continuum model method to compare with the microsolvation studies.