Structural, Energetic, and Infrared Spectra Insights into Methanol Clusters (CH3OH)n, for n = 2-12, 16, 20. ONIOM as an Efficient Method of Modeling Large Methanol Clusters.

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.