Theoretical study of structure and spectra of cage clusters (H2O)(n), n=7-10

The optimal structures, interaction energies, and OH stretch frequencies of water cage clusters, (H2O)(n), n = 7-10, have been investigated theoretica lly. The study consisted of MP2 level ab initio calculations of optimal str uctures and spectra and similar calculations employing an empirical polariz able potential. The study focuses predominantly on structures, that were us ed for assignment of recently measured size selected OH stretch spectra. Th e structures are related to octamer cubes. The nonamer and the decamer are derived from the octamer by insertion of one and two two-coordinated molecu les, respectively, into the D-2d cube edges. The two heptamer isomers are o btained by removal of a single water molecule from the Sq octamer cube. The n = 8-10 clusters correspond to relatively regular structures, characteriz ed by two distinct groups of O .. O bond lengths and corresponding stretch frequencies. The more strained and asymmetric heptamers include a broad ran ge of hydrogen bonded configurations, which are reflected by a complex OH s tretch spectrum with numerous peaks. Two additional low energy decamer stru ctures were investigated, which correspond to two fused pentamer rings. Fol lowing past suggestions, we further explored the possibility of parameteriz ing OH bond frequency as a function of the electric field at the H atom.