Dynamics and structure of disordered hydrogen-bonded crystals

Since the problem of the structure and of the molecular and lattice dynamics of solid methanol seems yet unsolved, new contributions are presented coming from theoretical calculations and lattice dynamical calculations. The α and β structures of solid methanol are accounted for on the basis of CNDO and packing-forces calculations. It turns out that a structurally disordered model for the β phase is likely to occur. A mechanism for the λ-type phase transition at 157·4° K is proposed which qualitatively accounts for the data from specific heat, dielectric constants, N.M.R. line-width and X-ray studies. The vibrational spectrum is discussed and compared with the interpretations available in the previous literature. Attempts are made to find evidence in the infra-red of the disordered model proposed from theoretical calculations. k = 0 phonons are calculated and their geometry dependence studied. While the O-H stretching motion is independent from an out-of-plane deformation of the molecular chain, O-H … O deformation modes are instead very sensitive to this variable. The calculated force constants give some useful information on the nature of the hydrogen bond.