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

Abstract

The water hexamer is known to have a number of isoenergetic structures. The first experimental identification of the O–H stretching vibrational spectra 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-rotational tunneling (VRT) spectroscopy. Although both experiments seem to have located only the Cage structure, the structure of the benzene–water hexamer complex is not clearly known, and the effect of benzene in the water hexamer is unclear. In particular, it is not obvious how the energy difference between nearly isoenergetic water hexamer conformers changes in the presence of benzene. Thus, we have compared the benzene complexes with four low-lying isoenergetic water hexamers, Ring, Book, Cage, and Prism structures, using ab initio calculations. We also investigated the effects of the presence of benzene on the structures, harmonic vibrational frequencies, and infrared (IR) intensities for the four low-lying energy conformers. There is little change in the structure of the water hexamer upon its interaction with the benzene molecule. Hence the deformation energies are very small. The dominant contribution to the benzene–water cluster interaction mainly comes from the π–H interactions between benzene and a single water molecule. As a result of this π–H interaction, O–Hπ bond length increases and the corresponding stretching vibrational frequencies are redshifted. The IR spectral features of both (H2O)6 and benzene–(H2O)6 are quite similar. From both the energetics and the comparison of calculated and experimental spectra 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 structures, only one conformer matches the experimental O–H vibrational frequencies.