Crystal Spectra in Polarized Light, Vibrational Assignment, and Force-Constant Calculations of Phenanthrene

Abstract

A complete vibrational assignment of phenanthrene C₁₄H₁₀ is presented. The assignment of the fundamental vibrations of this molecule is based on the following data: (i) a priori calculation of the normal frequencies using a force field including 27 force constants, (ii) infrared spectrum in polarized light of single crystal between 4000 and 80 cm⁻¹, (iii) Raman and fluorescence spectra of single crystals. The force field used in the present calculation was obtained from an ``overlay'' refinement of benzene, benzene‐d₆, naphthalene, naphthalene‐d₈, anthracene, and anthracene‐d₁₀. The interpretation of the crystal spectrum in polarized light was based on the use of the oriented gas model. It was found however that in some cases large deviations occur between the experimental and the theoretical dichroism. A discussion of the intermode mixing mechanism is included.