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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-H[sub π] bond length increases and the corresponding stretching vibrational frequencies are redshifted. The IR spectral features of both (H[sub 2]O)[sub 6] and benzene-(H[sub 2]O)[sub 6] are quite similar. From both the energetics and the comparison of calculated and experimental spectra of the benzene-(H[sub 2]O)[sub 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. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The correlation of various properties of water clusters (H[sub 2]O)[sub n=1-10] to the cluster size has been investigated using extensive ab initio calculations. Since the transition from two dimensional (2-D) (from the dimer to pentamer) to 3-D structures (for clusters larger than the hexamer) is reflected in the hexamer region, the hexamer can exist in a number of isoenergetic conformers. The wide-ranging zero-point vibrational effects of the water clusters having dangling H atoms on the conformational stability by the O-H flapping or proton tunneling through a small barrier (∼0.5 kcal/mol) between two different orientations of each dangling H atom are not large (∼0.1) kcal/mol). Large dipole moments (>2.5 D) are found in the dimer and decamer, and significant dipole moments (∼2 D) are observed in the monomer, hexamer, and nonamer. The polarization per unit monomer rapidly increases with an increasing size of the cluster. However, this increase tapers down beyond the tetramer. The O-H vibrational frequencies serve as sensitive indicators of the status of proton donation ("d") and acceptance ("a") (i.e., the structural signature of H-bond type) for each water monomer in the cluster. In general, the magnitudes of the O-H frequencies (ν) for each cluster can be arranged in the following order: ν[sub 3][sup da] (single donor-single acceptor) ≅ν[sub 3][sup daa] (single donor-double acceptor) >ν[sub 3][sup dda] (double donor-single acceptor) >ν[sub 1][sup dda]>ν[sub 1][sup da]> (or ≅) ν[sub 1][sup daa]. The increase in the cluster size has a pronounced effect on the decrease of the lower frequencies. However, there are small changes in the higher frequencies (ν[sub 3][sup da] and ν[sub 3][sup daa]). The intensities of ν[sub 1][sup daa] and ν[sub 1][sup da] are very high, since the increased atomic charges can be correlated to the enhanced H-bond relay effect. On the other hand, the intensities of the ν[... [ABSTRACT FROM AUTHOR]