Structural characterization of the (methanol)4 potential energy surface.

In this paper, we report the geometries and properties of the structural isomers obtained from a random walk of the potential energy surface (PES) of the methanol tetramer. Thirty-three structures were obtained after B3LYP/6-31+g* optimization of 94 candidate structures generated from a stochastic search of the PM3 conformational space. The random search was carried out using a recently proposed modified Metropolis acceptance test in the simulated annealing (SA) procedure. Corrections for the basis set superposition error (BSSE) show improvements on the binding energies of the clusters in an average of approximately 2.0 kcal/mol, while geometries are predicted to be less sensitive to BSSE corrections. MP2/aug-cc-pvdz calculations on representative structures did not change the geometries but predicted better binding energies. Highly correlated CCSD(T) energies were calculated on the B3LYP and MP2 stationary points and used to establish relative stabilities. We report several new conformations and group the structures into six distinct geometrical motifs. Only the cyclic tetramers with four primary hydrogen bonds in the same plane are predicted to have significant populations. Secondary hydrogen bonds, those for which the donated proton comes from an alkyl group, lead to a rich conformational space.