INTERACTION ENERGIES ASSOCIATED WITH SHORT INTERMOLECULAR CONTACTS OF C-H BONDS - ABINITIO COMPUTATIONAL STUDY OF THE C-H...O CONTACT INTERACTION IN CH4...OH2

To accurately evaluate the C-H...O contact interaction energy, ab initio calculations were performed on CH4...OH2 at the self-consistent-field molecular orbital (SCF-MO) and MP2 levels with various quality basis sets including a near Hartree-Fock limit (HFL) one. The interaction energies calculated with truncated basis sets have small basis set superposition errors and become close to the near HFL results only when they include diffuse functions. This is attributed to the necessity of properly representing the lone pair electrons of oxygen that make short contact interaction with the C-H bond. Our MP2 calculations suggest that the conformation of CH4...OH2 with one C-H...O contact is the only minimum-energy structure. At the MP2 level with the near HFL basis set, the binding energy of CH4...OH2 is calculated to be 0.59 +/- 0.05 kcal/mol. The potential energy curves of CH4...OH2 calculated as functions of the H...O contact angles are consistent with the observation that the most frequently found H...O contact angles of organic solids are in the range \phi\ less-than-or-equal-to approximately 60-degrees and \theta\ less-than-or-equal-to approximately 30-degrees.