σ to π conformational transition: Interactions of the water trimer with π systems

We investigate the interaction of the water trimer with the aromatic π systems (benzene, toluene, fluorobenzene, and p-difluorobenzene) at the second-order Møller–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 π type of interaction with benzene and toluene, but has both a π and σ 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 π 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 π and σ type of interaction observed in the fluorobenzene and p-difluorobenzene complexes. Thus one observes that the OH red-shift associated with the π type of interaction is larger than with the σ 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 π type of interaction over the σ type of interaction. This inference is also supported by the magnitude of the two-body terms associated with the π and σ type of interaction in both the fluorobenzene and p-difluorobenzene complexes.