Lattice Dynamics of Methanol: Hydrogen Bonding and Infrared Absorption

Mathematical methods of treating lattice dynamics of solids have been applied to interpret the infrared spectrum of crystalline CH3OH and CH3OD. New spectra were recorded in the near-infrared region for crystalline CH3OH, CH3OD, and of mixed crystals at various concentrations. The far-infrared spectrum is also reported. Attempts were made to find spectral evidence of the two crystalline modifications of CH3OH. A valence force field has been derived by a least squares fitting on the k = 0 modes of both molecules; the assignments of fundamental frequencies are discussed. Based on this derived force field, phonon dispersion curves and one-phonon and two-phonon densities of states have been calculated in the single-chain approximation. These are used in the interpretation of the infrared spectrum. The vibrational spectra of mixed crystals of CH3OH and CH3OD reveal unusual features which suggest that the breadth of absorption bands due to the hydrogen-bonded atoms is not a property of a single hydrogen bond but arises from cooperative effects with similar neighbors in the chain. The broadness seems to be a function of the chain length.

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