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 minimum energy structure has almost D2d symmetry, with an approximately cubical oxygen framework and four tetrahedrally arranged free hydrogens; four of the water molecules are single- and four are double-hydrogen donors. 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 clusters. The O–H stretching frequencies of single- and double-hydrogen donor water molecules are relatively well separated from each other, and both from the frequency region of the free O–H stretches, suggesting a new interpretation for some of the data. The low-frequency translational/librational modes of both n=5 and 8 show strong mixing with intramolecular stretching and bending. The stretch–stretch coupling constants for OH oscillators on different molecules kij(OH,OH) show a strong increase, and those for intramolecular coupling kii(OH,OH) a rapid decrease with increasing cluster size. For n≥5, kij(OH,OH)≫kii(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.