Infrared spectra of large H2O clusters: New understanding of the elusive bending mode of ice

Infrared spectroscopic data for large water clusters, ranging from similar to 100 to 64 000 molecules, have been extended to the O-H stretch and bendi ng mode regions of H2O. The stretch-mode spectra and analysis parallel resu lts reported recently for D2O large clusters. Ambiguities in the current un derstanding of the bending region of the spectra of amorphous and crystalli ne ice are addressed using insights derived from the cluster infrared spect ra and ab initio reduced-dimensionality models of ice and of the ice surfac e. The 1400-1700 cm(-1) spectral range, for annealed amorphous ice and crys talline cubic ice at 10 K, is characterized by a broad absorption lacking d istinct features assignable to water bending vibrations. The spectra sugges t that the bend-mode intensity of the bulk ices is either inherently very w eak, diffused through interaction with the librational overtone, or both. H owever, this is not the case for large water clusters, which display distin ct and relatively narrow bands attributed to the bending modes of subsets o f surface water molecules. The new computational results suggest a strong b ut irregular dependence of the bend-mode frequency and band intensity on th e strength and tetrahedrality of the hydrogen bonding. The computed intensi ty does decrease substantially for 4-coordinated vs lower-coordinated water s, but the highest frequency corresponds to double-donor 3-coordinated surf ace water molecules. A significant decrease of the librational-mode frequen cy, for those cases for which hydrogen bonding is reduced relative to ice I , is known to reduce the overlap of the bending mode with the librational o vertone. Thus, for liquid water, microporous amorphous ice and the ice surf ace, which deviate strongly from tetrahedral bonding and for which the hydr ogen bonding is diminished, the bend-mode absorption assumes a more normal intensity and bandwidth. From this basis a qualitative interpretation is pr esented of the infrared spectra for the region from similar to 500 to simil ar to 2400 cm(-1).