STRUCTURES AND VIBRATIONAL-SPECTRA OF WATER CLUSTERS IN THE SELF-CONSISTENT-FIELD APPROXIMATION

Fully optimized structures were calculated for (H2O)n, n = 5 and 8, at the SCF (self-consistent field) level using the 4-31G and, for n = 5, also 6-31G basis sets. The n = 5 cluster was found to have a cyclic structure with five H bonded and five free hydrogens. The n = 8 minimu m energy structure has almost D2d symmetry, with an approximately cubi cal oxygen framework and four tetrahedrally arranged free hydrogens; f our of the water molecules are single- and four are double-hydrogen do nors. Harmonic vibrational frequencies, IR and Raman intensities were calculated for n = 5 and 8, as well as for the previously optimized n = 2-4 clusters. The band positions and intensities in the 3000-3800 cm -1 region correlate well with IR predissociation spectra of (H2O)n clu sters. The O-H stretching frequencies of single- and double-hydrogen d onor water molecules are relatively well separated from each other, an d both from the frequency region of the free O-H stretches, suggesting a new interpretation for some of the data. The low-frequency translat ional/librational modes of both n = 5 and 8 show strong mixing with in tramolecular stretching and bending. The stretch-stretch coupling cons tants for OH oscillators on different molecules k(ij) (OH,OH) show a s trong increase, and those for intramolecular coupling k(ii) (OH,OH) a rapid decrease with increasing cluster size. For n greater-than-or-equ al-to 5, k(ij) (OH,OH) >> k(ii) (OH,OH), implying that the cluster can be viewed as a supermolecule of strongly coupled O-H oscillators. The n = 8 spectra show significant similarity to those of ice.