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 theoretically. The study consisted of MP2 level ab initio calculations of optimal structures and spectra and similar calculations employing an empirical polarizable potential. The study focuses predominantly on structures, that were used for assignment of recently measured size selected OH stretch spectra. The structures are related to octamer cubes. The nonamer and the decamer are derived from the octamer by insertion of one and two two-coordinated molecules, respectively, into the D-2d cube edges. The two heptamer isomers are obtained by removal of a single water molecule from the Sq octamer cube. The n = 8-10 clusters correspond to relatively regular structures, characterized by two distinct groups of O .. O bond lengths and corresponding stretch frequencies. The more strained and asymmetric heptamers include a broad range of hydrogen bonded configurations, which are reflected by a complex OH stretch spectrum with numerous peaks. Two additional low energy decamer structures were investigated, which correspond to two fused pentamer rings. Following past suggestions, we further explored the possibility of parameterizing OH bond frequency as a function of the electric field at the H atom.