Ab initio calculations of cooperativity effects on clusters of methanol, ethanol, 1-propanol and methanethiol

The results of ab initio calculations for cyclic clusters of methanol, etha nol, I-propanol, and methanethiol are presented. Dimer, trimer, and tetrame r clusters of all four compounds are studied, as are pentamer and hexamer c lusters of methanol. From optimized clusters at HF/6-31G**, total energies and binding energies were calculated with both the HF and MP2 theories usin g the aug-cc-pVDZ basis set. Accurate binding energies were also calculated for the dimer and trimer of methanol using symmetry-adapted perturbation t heory with the same basis set. Intermolecular and intramolecular distances, charge distribution of binding sites, binding energies, and equilibrium co nstants were computed to determine the hydrogen bond cooperativity effect f or each species. The cooperativity effect, exclusive to hydrogen bonding sy stems, results from specific forces among the molecules, in particular char ge-transfer processes and the greater importance of interactions between mo lecules not directly hydrogen bonded because of the longer range of the int eractions, The ratios of equilibrium constants for forming multimer hydroge n bonds to that for dimer hydrogen bond formation increase rapidly with the cluster size, in contrast to the constant Value commonly used in thermodyn amic models for hydrogen bonding liquids.