Molecular bending mode frequencies of the surface and interior of crystalline ice

Abstract

The Fourier transform infrared (FT-IR) spectra of the bending modes of the three categories of molecules at the surface of ice nanocrystals have been determined for both H2O and D2O samples and the response of the bending mode of the dangling hydrogen surface molecules to association with adsorbates of varying acceptor strengths has been examined. From these combined data for water molecules in a wide range of environments at the ice surface, a clear picture of the dependence of the bending mode frequency on the extent of H-bonding is revealed for ice-related conditions. These frequencies have been examined in terms of the linear equation developed by Falk that relates the water decoupled bend mode frequency to H-bond strength through the average of the symmetric and asymmetric stretch mode frequencies. The published value of 1735 cm−1, for the decoupled bend mode frequency of bulk H2O ice, is consistent with this presentation of the new data, but a new value, higher by at least 40 cm−1 than the accepted value of ∼1220 cm−1, is indicated for D2O ice. A value of 1265 cm−1 is offered as the best estimate of the bend mode frequency of D2O ice.