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An incoherent inelastic neutron scattering experiment on polycrystalline ice Ih at T=20 K is reported. The experiment was performed on the high-resolution chopper spectrometer at the Intense Pulsed Neutron Source (IPNS) of the Argonne National Laboratory. Scattering functions in the angular range from 4° to 20° were converted, after analytical corrections, into density of states of the system in the stretching frequency region. Analysis of these results leads to the following conclusions: (i) the number of modes in the stretching region of ice Ih is 8.1/cell which is that expected in the case of free molecules; (ii) the general shape of the density of states is in qualitative agreement with Raman scattering results, however the high energy side of the neutron data suggests that a non-Raman active band exists at about 3500 cm−1; (iii) the overall bandwidth observed in this experiment is in good agreement with what we estimated by a coherent inelastic neutron scattering study on heavy ice Ih.

In order to explain the observed changes in the vibrational spectra of formic acid from monomeric to cyclic dimeric form the SCF Hartree–Fock method has been applied to calculate minimum energy geometries, quadratic force constants, and net atomic charges. The set of ab initio quadratic force constants turns out to be acceptable and accounts for the observed vibrational changes. The ab initio interaction force constant between carbonyl stretching motions for the dimer accounts for what is considered to be an unusually large observed splitting of the C=O stretching frequencies in carboxylic acid dimers. Calculations suggest that such a large splitting originates from a dynamical charge transfer through the hydrogen bond. The possible existence of this phenomenon should in general be considered when dealing with the vibrational properties of hydrogen bonded systems.

It is experimentally known that ice Ih is orientationally disordered. The vibrational spectrum has been previously interpreted by other authors in terms of a simplified point mass model. On the basis of this model long-range forces, spectral activation by the disorder, fluctuating ordered polar domains, etc., were proposed. We present in this paper the results of a lattice dynamical treatment on a complete ordered model of ice Ih which includes the hydrogen atoms. A valence force field has been derived from optical and neutron spectra of H2O and D2O. The introduction of all the vibrational degrees of freedom and their possible interactions does affect the interpretation of the spectrum and the derived spectral conclusions. The interpretations of the spectral evidences on which the previous works were based are discussed.

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