HYDROGEN-BOND SWAPPING IN THE BENZENE WATER COMPLEX - A MODEL STUDY OF THE INTERACTION POTENTIAL

Model calculations have been carried out to characterize the detailed nature of the benzene-water intermolecular interaction potential energy surface. At equilibrium, the calculations show a binding energy of 1303 cm-1. The optimum water molecule's position is above the plane of the benzene ring and shifted from benzene's symmetry axis by 0.82 angstrom. Of course, the equilibrium position alone does not satisfactorily characterize the benzene-water interactions because of the extreme floppiness of the complex. The calculations indicate that even at the zero-point level, the water molecule is capable of two very-low-energy routes interconverting between indistinguishable structures which involve water motions over large fractions of the benzene ring. One is the nearly free internal rotation of the benzene ring. The other is a combined translation and torsion of the water molecule where the hydrogens of the water molecule are exchanged, a process that is really a swapping or interchange of hydrogen bonds. The reason for its remarkably-low-energy barrier is discussed along with the connection between the surface features and experimental observations.