Ab initio studies of π-water tetramer complexes:: Evolution of optimal structures, binding energies, and vibrational spectra of π-(H2O)n (n=1-4) complexes

The optimal structures, binding energies, and harmonic vibrational frequencies of clusters containing a substituted benzene molecule microsolvated by four water molecules, termed as pi-(water tetramer) clusters (pi: p-difluorobenzene, fluorobenzene, benzene, toluene) have been evaluated at the second order perturbation level of theory (MP2) using both the 6-31+G* and aug-cc-pVDZ basis sets. In sharp contrast to the complexes of smaller water clusters with these pi systems, wherein the water subcluster is most strongly bound to toluene, the water tetramer is most strongly bound to fluorobenzene. This exceptionally high binding energy results from both a pi . . . OH H-bond and a competing sigma F . . . OH bond between the water tetramer moiety and the aromatic molecule. The magnitudes of the many-body energy terms and their contribution to the binding energies of these pi-(water tetramer) systems indicates that the contributions of three- and higher-order terms are much smaller when compared to the neutral water clusters. The two-body terms associated with the pi- and sigma -type of interaction indicates that in both the fluorobenzene and p-difluorobenzene complexes, the increase in the size of the water cluster enhances the pi -H-bonding interaction and weakens the sigma F . . .H interaction. This observation is in consonance with the calculated and experimentally observed redshifts of the OH vibrational frequencies. Thus, with an increase in the size of a water cluster bound to the fluorinated pi system, there is a lowering of the redshift induced by the sigma F . . .H interaction and an increase in the redshift due to the pi -H interaction. The calculated redshift of the pi H-bonded OH mode is very much dependent on the basis set, with larger basis sets yielding shifts which are in better agreement with the experimentally determined shifts. (C) 2001 American Institute of Physics.