Interpretation of the Infrared Spectrum of a Molecular Crystal: Naphthalene

Abstract

A critical examination of the oriented gas model is made on the basis of several suggested criteria applied to the infrared studies of solid C₁₀H₈ and C₁₀D₈. The frequency difference between gas and solid spectra is as high as 1 percent ν for only a few naphthalene bands. The dichroic band splittings which can be assigned to intermolecular interactions are small, averaging less than 3 cm^—1, but with a few well defined exceptions. A comparison between the observed and predicted dichroic behaviors for those bands whose symmetry classes are known indicates that the dichroism is dominated by the gas phase symmetries of the vibrational modes. The general applicability of the oriented gas model for naphthalene indicates its usefulness in interpreting those solid spectra. On the other hand, the occasional, distinct deviations from the model warn that it is not likely to be useful in studying crystals involving stronger crystal forces (such as hydrogen bonds or ionic forces).