A Theoretical Investigation Of The Nature Of The Pi-H Interaction In Ethene-H2o, Benzene-H2o, And Benzene-(H2o)(2)

We have carried out a detailed investigation of the nature of the pi-H interaction in the ethene-H2O, benzene-H2O, and benzene-(H2O)(2) complexes using large basis sets (ranging from 6-31+G* to TZ2P++) and high levels of theory. The minimum geometries, and hence the vibrational frequencies, of all the complexes have been obtained at the second order Moller-Plesset (MP2) level of theory. The binding energy of the ethene-H2O complex is only about 1 kcal/mol lower than that of the benzene-H2O complex. In the benzene-(H2O)(2) complex, the interaction of benzene with the pi-bonded water to that with the second water is nearly equivalent. In order to explain the above interesting facets of the interaction of water with benzene and ethene, the interaction energies were decomposed into the individual interaction energy components using the recently developed symmetry adapted perturbation theory (SAPT) program. The SAPT results indicate that the repulsive exchange energies play a crucial role in governing the energies and geometric preferences of these complexes. A detailed analysis of the vibrational frequencies of these complexes has also been done to examine the changes in the frequencies of the monomers upon complexation. It is found that changes in the out-of-plane bending frequencies of benzene and ethene can be correlated to the interaction energies of these complexes, in particular the exchange energy. (C) 1999 American Institute of Physics. [S0021-9606(99)30337-8].