Benzene-hydrogen halide interactions: Theoretical studies of binding energies, vibrational frequencies, and equilibrium structures

High level ab initio calculations have been performed on the benzene-HCl and benzene-HF systems using the second-order Moller-Plesset perturbation theory. In contrast to existing theoretical studies, the calculated binding energies indicate that HCl binds more strongly to benzene than HF. This is in accordance with the limited experimental data available on these systems. An explanation has been forwarded for the above observation by performing a molecular orbital analysis of both C6H6 ... HF and C6H6 ... HCl In the global minimum of C6H6 ... HF, HF lies inclined to the benzene ring with the hydrogen atom pointing either towards a benzene carbon or the center of carbon-carbon bond. In the C6H6 ... HCl complex, HCl is found to lie along the C-6 axis of the benzene ring for smaller basis sets, but it also tends to lie inclined to the benzene ring for a very large basis set. The quantum mechanical probabilistic characterization of the structure of the C6H6 ... HCl complex provides a more realistic description of the experimental equilibrium structure. The van der Waals modes have also been characterized, and the modulation of these modes as one progresses from HF to HCl has also been studied. (C) 1998 American Institute of Physics.