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

The optimal structures, binding energies, and harmonic vibrational frequenc ies of clusters containing a substituted benzene molecule microsolvated by four water molecules, termed as pi-(water tetramer) clusters (pi: p-difluor obenzene, fluorobenzene, benzene, toluene) have been evaluated at the secon d order perturbation level of theory (MP2) using both the 6-31+G* and aug-c c-pVDZ basis sets. In sharp contrast to the complexes of smaller water clus ters with these pi systems, wherein the water subcluster is most strongly b ound 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 moiet y and the aromatic molecule. The magnitudes of the many-body energy terms a nd their contribution to the binding energies of these pi-(water tetramer) systems indicates that the contributions of three- and higher-order terms a re much smaller when compared to the neutral water clusters. The two-body t erms 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 we akens the sigma F . . .H interaction. This observation is in consonance wit h the calculated and experimentally observed redshifts of the OH vibrationa l 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 b y 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 v ery 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.