Infrared Spectra of Large H2O Clusters:  New Understanding of the Elusive Bending Mode of Ice

Infrared spectroscopic data for large water clusters, ranging from ∼100 to 64 000 molecules, have been extended to the O−H stretch and bending mode regions of H2O. The stretch-mode spectra and analysis parallel results reported recently for D2O large clusters. Ambiguities in the current understanding of the bending region of the spectra of amorphous and crystalline ice are addressed using insights derived from the cluster infrared spectra and ab initio reduced-dimensionality models of ice and of the ice surface. The 1400−1700 cm-1 spectral range, for annealed amorphous ice and crystalline cubic ice at 10 K, is characterized by a broad absorption lacking distinct features assignable to water bending vibrations. The spectra suggest that the bend-mode intensity of the bulk ices is either inherently very weak, diffused through interaction with the librational overtone, or both. However, this is not the case for large water clusters, which display distinct and relatively narrow bands attributed to the bending modes of subsets of surface water molecules. The new computational results suggest a strong but irregular dependence of the bend-mode frequency and band intensity on the strength and tetrahedrality of the hydrogen bonding. The computed intensity does decrease substantially for 4-coordinated vs lower-coordinated waters, but the highest frequency corresponds to double-donor 3-coordinated surface water molecules. A significant decrease of the librational-mode frequency, 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 overtone. Thus, for liquid water, microporous amorphous ice and the ice surface, which deviate strongly from tetrahedral bonding and for which the hydrogen bonding is diminished, the bend-mode absorption assumes a more normal intensity and bandwidth. From this basis a qualitative interpretation is presented of the infrared spectra for the region from ∼500 to ∼2400 cm-1.