Structures, vibrational frequencies, and infrared spectra of the hexa-hydrated benzene clusters

The water hexamer is known to have a number of isoenergetic structures. The first experimental identification of the O-H stretching vibrational spectr a of the water hexamer was done in the presence of benzene. It was followed by the identification of the pure water hexamer structure by vibration-rot ational tunneling (VRT) spectroscopy. Although both experiments seem to hav e located only the Cage structure, the structure of the benzene-water hexam er complex is not clearly known, and the effect of benzene in the water hex amer is unclear. In particular, it is not obvious how the energy difference between nearly isoenergetic water hexamer conformers changes in the presen ce of benzene. Thus, we have compared the benzene complexes with four low-l ying isoenergetic water hexamers, Ring, Book, Cage, and Prism structures, u sing ab initio calculations. We also investigated the effects of the presen ce of benzene on the structures, harmonic vibrational frequencies, and infr ared (IR) intensities for the four low-lying energy conformers. There is li ttle change in the structure of the water hexamer upon its interaction with the benzene molecule. Hence the deformation energies are very small. The d ominant contribution to the benzene-water cluster interaction mainly comes from the pi-H interactions between benzene and a single water molecule. As a result of this pi-H interaction, O-H-pi bond length increases and the cor responding stretching vibrational frequencies are redshifted. The IR spectr al features of both (H2O)(6) and benzene-(H2O)(6) are quite similar. From b oth the energetics and the comparison of calculated and experimental spectr a of the benzene-(H2O)(6), the water structure in these complexes is found to have the Cage form. In particular, among the four different Cage structu res, only one conformer matches the experimental O-H vibrational frequencie s. (C) 2000 American Institute of Physics. [S0021-9606(00)31039-X].