STRUCTURAL AND DYNAMIC PROPERTIES OF HYDROGEN-BONDING IN A TETRAHEDRAL ARRANGEMENT OF METHANOL MOLECULES - A THEORETICAL INVESTIGATION

The crystal structure of triphenylmethanol contains tetramers of triph enylmethanol molecules, in which the four oxygen atoms are arranged ap proximately at the comers of a tetrahedron with the C-O bonds along th e threefold symmetry axes of the tetrahedron. The O ... O distances in the tetrahedron are consistent with the suggestion that the tetramer is held together by 0-H ... 0 hydrogen bonding. To understand the stru ctural and dynamic properties of this arrangement, we consider a model system comprising a structurally analogous tetramer of methanol molec ules. In the theoretical approach adopted, hydrogen bonding is treated as a purely electrostatic interaction, with the molecular charge dist ributions described using distributed multipoles. The optimum position s of the hydrogen atoms involved in hydrogen bonding have been determi ned, and dynamic properties of the hydrogen bonding arrangement have b een probed. In the configuration with lowest electrostatic energy, eac h oxygen atom of the methanol tetramer receives one hydrogen bond, wit h the hydrogen atoms lying close to the 0 ... 0 edges of the tetrahedr on. Dynamics of the hydrogen bonding arrangement, via rotation of the methanol molecules about their C-0 bonds, involves correlated rotation of all four methanol molecules to interconvert configurations of equa l energy. The barrier for this motion is approximately 5 kJ mol-1.