Intramolecular vibrations of small water clusters

Abstract

The structures, binding energies, and vibrational frequencies of fully optimized water clusters (H2O)n, n=1-4, were computed with ab initio molecular orbital theory at the SCF level using different basis sets. The SCF procedure allows satisfactory predictions for these properties compared with experimental results. For quantitative predictions of binding energies and geometrical parameters, a basis set including polarization functions is needed. With respect to intramolecular vibrational frequencies and frequency shifts, however, the split valence basis set 4-31G leads in all cases to the best rationalization of the available experimental data. Analysis of intramolecular force constants, frequencies, and eigenvectors for n=2 to 4 shows that (i) a transition from highly localized (n=2) to highly delocalized (n=4) vibrational modes takes place; (ii) the delocalized O-H vibrations of cyclic (H 2O)n clusters (n≥3) can be described as longitudinal/transverse optical phonons; (iii) internal force constants for the hydrogen bonding O-H stretch, k(O-Hb), decrease strongly with size, leading to a decrease in the νbridge O-H frequencies; a similar decrease is found for the intramolecular bending force constant k(Θ); (iv) force constants for the free O-H stretching motions k(O-Hf) stay nearly constant, as do the corresponding vibrational frequencies; (v) intermolecular stretch-stretch couplings increase substantially with n and dominate over intramolecular stretch-stretch couplings for n≥3; similarly, intermolecular bend-libration couplings are very important. © 1988 American Institute of Physics.