Infra-red spectrum of CH₃F at very high resolution : Accidental and essential resonances in ν₂ + ν₃ and ν₃ + ν₅ bands

Abstract

The analysis of the Fourier transform spectrum, with a resolution of 0·008 cm^-1, of the combination bands ν₂ + ν₃ (A₁ species) and ν₃ + ν₅ (E species) of CH₃F has allowed the study of x-y Coriolis resonance between the states υ₂ = 1 and υ₅ = 1. The vibrational levels υ₂ = υ₃ = 1 and υ₃ = υ₅ = 1 are 15 cm^-1 apart and the interaction is very strong. The combination of the Coriolis resonance and the essential interaction of l-type (2, 2) in ν₃ + ν₅ gives rise to a ‘giant’ l-type doubling : for J=25 the shift between the levels is about 17 cm^-1.

A second essential interaction exists in the perpendicular band: the rotational l-type (2, -1) resonance. The effect of this is small, but the addition of this interaction causes a chain of resonances between ro-vibrational levels and gives rise to a doubling of ^PP₃(J) transitions in the perpendicular band.

Molecular parameters and coupling constants have been derived.