The infra-red spectrum of ¹³CH₃F and the general harmonic force field of methyl fluoride

Abstract

Analyses of the v ₃, 2v ₃, and (predominantly) v ₁ parallel bands, and of the v ₄ and v ₆ perpendicular fundamentals have been made for ¹³CH₃F in terms of the rotational structure observed with a resolution of ∼0·2 cm^-1. In addition, the band centres of the strongly Coriolis-interacting v ₂ and v ₅ fundamentals are accurately located. Some elucidation of the complex Fermi resonance interactions in the 3000 cm^-1 region is achieved through study of spectra of crystalline samples. This enables all three components of the v ₁, 2v ₂, 2v ₅ ⁰ triad to be observed for both ¹²CH₃F and ¹³CH₃F, and estimates to be made for the unperturbed vibration frequencies.

The ¹³C frequency shifts determined for all six fundamentals are used in conjunction with existing frequency, Coriolis ζ, and centrifugal distortion data for CH₃F, CD₃F, CHD₂F and CH₂DF, to determine the general harmonic force field for methyl fluoride. The extra shift data enable all 12 parameters of the force field to be fixed within narrow limits for the first time. The disagreement with predictions of the hybrid orbital model in the A₁ species can be attributed to the effect of trans repulsions arising from the fluorine lone-pair electrons, an effect which contributes to the longer CH bond in methyl fluoride compared with the other methyl halides.