Interaction of the water dimer with pi-systems: A theoretical investigation of structures, energies, and vibrational frequencies

The interaction of the water dimer with both the olefinic and aromatic pi s ystems (ethene, benzene, toluene, fluorobenzene, and p-difluorobenzene) has been investigated using both the supermolecular [second-order Moller-Pless et (MP2)] and perturbational (symmetry adapted perturbation theory) approac hes. The geometry optimizations, harmonic vibrational frequencies, and the components of the binding energy were evaluated using fairly large basis se ts (6-31 + G* and aug-cc-pVDZ). The minimum energy structures obtained at t he MP2/6-31 + G* and MP2/aug-cc-pVDZ levels of theory indicate that the wat er dimer exhibits a pi-type of interaction with ethene, benzene, and toluen e and a sigma-type of interaction with both fluorobenzene and p-difluoroben zene. This is demonstrated from the vibrational frequencies which are in go od agreement with the experimentally determined numbers. Our calculations i ndicate that the nature and strength of the interaction of the donor water molecule (water dimer) with the pi system has a significant bearing on the total binding energy of the complex. Apart from the interaction of the wate r dimer with the pi system, we also show how this interaction influences th e hydrogen bond characteristics of the water dimer by evaluating the hydrog en bond strengths. (C) 2000 American Institute of Physics. [S0021-9606(00)3 0604-3].