An improved analysis of the OH stretching region of the vibrational spectrum of ice Ih

Abstract

We report the results of an analysis of the OH stretching region of the vibrational spectrum of ice Ih. The model of ice Ih discussed in an earlier paper [J. Chem. Phys. 69, 3483 (1978)] is considerably improved in that a larger section of the proton disordered lattice is included in the dynamical description, the shift in the OH stretching force constant on hydrogen bonding is calculated from molecular properties, the interaction force constants are re-evaluated using newer and more extensive data, and Fermi resonance and internal field effects on the lattice dynamics are accounted for. We show the following: (i) Strong coupling between OH oscillators on different molecules is responsible for the major features of the vibrational spectrum, namely, its breadth and gross distribution of intensity. (ii) Internal field effects, which lead to wave vector dependent polar mode mixing in a proton ordered model of ice I, do not influence the distribution of intensity in the Raman spectrum of proton disordered ice Ih, despite the fact that the internal field does play an important role in altering the states of the system. Our calculations provide no support for the suggestion that structure in the Raman spectrum of ice Ih can be assigned using the Lyddane–Sachs–Teller relation, i.e., that structure in the Raman spectrum can be related to system modes arising under the influence of the longitudinal field. (iii) Fermi resonance between the OH stretching mode and the overtone of the bending mode has more influence on the vibrational spectrum of D2O ice Ih than on that of H2O ice Ih. The inclusion of Fermi resonance in the description of the OD stretching dynamics materially improves the agreement between the predicted and observed vibrational spectra. However, the redistribution of spectral intensity attributable to Fermi resonance is a second order effect relative to the distribution of intensity determined by the interaction between OH oscillators on different molecules. (iv) The change in OH stretching frequency when a water molecule is incorporated in ice Ih can be accounted for self-consistently using only the known free molecule anharmonic potential energy coefficients and a parametric description of the dependence of the harmonic force constants on the strength of hydrogen bonding. This finding validates and provides an interpretation of the quasiharmonic representation of OH stretching motions used in our earlier work.