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 triphenylmethanol molecules, in which the four oxygen atoms are arranged approximately at the comers of a tetrahedron with the C-O bonds along the 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 structural and dynamic properties of this arrangement, we consider a model system comprising a structurally analogous tetramer of methanol molecules. In the theoretical approach adopted, hydrogen bonding is treated as a purely electrostatic interaction, with the molecular charge distributions described using distributed multipoles. The optimum positions of the hydrogen atoms involved in hydrogen bonding have been determined, and dynamic properties of the hydrogen bonding arrangement have been probed. In the configuration with lowest electrostatic energy, each oxygen atom of the methanol tetramer receives one hydrogen bond, with the hydrogen atoms lying close to the 0 ... 0 edges of the tetrahedron. 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 equal energy. The barrier for this motion is approximately 5 kJ mol-1.