sigma to pi conformational transition: Interactions of the water trimer with pi systems

Abstract

We investigate the interaction of the water trimer with the aromatic pi systems (benzene, toluene, fluorobenzene, and p-difluorobenzene) at the second-order Moller-Plesset level of theory using both the 6-31+G* and aug-cc-pVDZ basis sets. The minimum-energy structures, binding energies, and the harmonic vibrational frequencies were calculated for all these systems. The minimum-energy structures reveal that the water trimer has only a pi type of interaction with benzene and toluene, but has both a pi and sigma type (H-bond with the fluorine atom) of interaction with fluorobenzene and p-difluorobenzene. The binding energies reveal that, contrary to what has been observed in previous theoretical studies, the strength of the interaction of these pi systems with the water trimer is greater than with the water dimer. A good agreement is obtained between the calculated and experimentally measured vibrational frequencies of these complexes. An analysis of the calculated OH stretching modes reveals some unusual facets of the pi and sigma type of interaction observed in the fluorobenzene and p-difluorobenzene complexes. Thus one observes that the OH red-shift associated with the pi type of interaction is larger than with the sigma type of interaction. This is particularly interesting because in both the water monomer and water dimer complexes, the OH red-shift associated with the interaction of the water cluster with fluorobenzene and p-difluorobenzene is due to the formation of the H-bond with the fluorine atom. Thus the increase in the size of the water cluster seems to favor the pi type of interaction over the sigma type of interaction. This inference is also supported by the magnitude of the two-body terms associated with the pi and sigma type of interaction in both the fluorobenzene and p-difluorobenzene complexes. (C) 2001 American Institute of Physics.