Structural, energetic, and infrared spectra insights into methanol clusters (CH\u3csub\u3e3\u3c/sub\u3eOH)\u3csub\u3en\u3c/sub\u3e, for n = 2-12, 16, 20. ONIOM as an efficient method of modeling large methanol clusters

An investigation of gas-phase methanol clusters (CH OH) , 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. © 2007 American Chemical Society. 3

Anchoring the gas-phase acidity scale

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 (CH O-H) = 437.7 ± 2.8 kJ/mol, D (C H O-H) = 438.1 ± 3.3 kJ/mol, D ((CH ) CHO-H) = 442.3 ± 2.8 kJ/mol, D ((CH ) CO-H) = 444.9 ± 2.8 kJ/mol, D (C H -H) = 463.0 ± 2.7kJ/mol, and D (C H BH) = 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. 298 3 298 2 5 298 3 2 298 3 3 298 2 3 298 6

Absolute Rate Calculations: Atom and Proton Transfers in Hydrogen-Bonded Systems13

We calculate energy barriers of atom- and proton-transfer reactions in hydrogen-bonded complexes in the gas phase. Our calculations do not involve adjustable parameters and are based on bond-dissociation energies, ionization potentials, electron affinities, bond lengths, and vibration frequencies of the reactive bonds. The calculated barriers are in agreement with experimental data and high-level ab initio calculations. We relate the height of the barrier with the molecular properties of the reactants and complexes. The structure of complexes with strong hydrogen bonds approaches that of the transition state, and substantially reduces the barrier height. We calculate the hydrogen-abstraction rates in H-bonded systems using the transition-state theory with the semiclassical correction for tunneling, and show that they are in excellent agreement with the experimental data. H-bonding leads to an increase in tunneling corrections at room temperature

Comparison of gas-phase acidities of some carbon acids with their rates of hydron exchange in methanolic methoxide

Hydron exchange reaction rates, k M s , using methanolic sodium methoxide are compared with gas-phase acidities, ΔG kcal/mol, for four 9-YPhenylfluorenes-9- H, seven YC H C H(CF ) , seven YC H -C HClCF , and C F H. Fourteen of the fluorinated benzylic compounds and pentafluorobenzene result in near unity experimental hydrogen isotope effects that suggest substantial amounts of internal return associated with the exchange process. Although the reactions of 9-phenylfluorene have experimental isotope effects that appear to be normal in value, they do not obey the Swain-Schaad relationship. This suggests that they occur with small amounts of internal return. The entropies of activation, ΔS‡, are +12 to +14 eu, for the benzylic compounds and different significantly from those for the 9-YPhenylfluorenes, ΔS‡ of -8 to -12 eu. The ΔS‡ ∼ 1 eu for the reactions of pentafluorobenzene falls between the other compounds. Density functional calculations using B3LYP/6-31+G(d,p) are reported for the reactions of CH O (HOCH ) with C F H, C H CH(CF ) , C H CHlCF , and 9-phenylfluorene. Copyright © 2006 John Wiley & Sons, Ltd. exch Acid 6 4 3 2 6 4 3 6 5 3 3 3 6 5 6 5 3 2 6 5 3 -1 -1 0 i i i i