Vibrational Exciton Splitting, Fermi Resonance, and Crystal Structure of Methyl Iodide

Abstract

The splitting of the v₅ vibrational band in crystalline methyl iodide has played an important role in the interpretation of vibrational exciton splittings in molecular crystals. According to an earlier interpretation, the site‐group splitting was said to be much larger than the factor‐group (exciton) splitting, and the effect of isotopic substitution on the exciton splittings was said to be substantial. Evidence is given in this paper to show that this interpretation is questionable and that actually certain features in the v₅ structure are most likely caused by Fermi resonance between v₅ and the combination band v₃+v₆. The problem of mixed exciton and Fermi interactions is discussed and it is shown that the factor‐group splitting is divided over the Fermi components in a very simple way. The resulting exciton splitting of v₅ is 10.5 cm⁻¹ in CH₃I and 10 cm⁻¹ in CD₃I. The earlier interpretation of the crystal spectrum of methyl iodide had been used as a criterion for the rejection of the otherwise plausible crystal structure (C_2v¹²). Both spectral evidence and packing considerations are now consistent with the existence of isomorphous crystal structures for methyl chloride, methyl bromide, and methyl iodide, having space group C_2v¹² (Cmc2₁) with two molecules per primitive unit cell located at C_s sites.