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

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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 (C2v12). 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 C2v12 (Cmc21) with two molecules per primitive unit cell located at Cs sites.

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