The 2ν3/ν2/ν5/ν3 + ν6 band system of CH3Br revisited: Anharmonic and Coriolis interactions in a four-level system near 1400 cm−1

The infrared spectrum of gaseous methyl bromide (CH₃Br) in natural abundance has been analyzed with high resolution, between 1150 and 1700 cm⁻¹. In this spectral region, two fundamental bands, ν₂ (A₁) and ν₅ (E), an overtone 2ν₃ (A₁) and a combination band ν₃ + ν₆ (E) occur. The novelty of the present study consists in considering, besides the strong Coriolis and α-interactions coupling the v₂ = 1 and v₅ = 1 levels, a large variety of anharmonic and rovibrational interactions involving also the v₃ = 2 and v₃ = v₆ = 1 levels. For this latter level, the data set belonging to the ν₃ + ν₆ combination band has been extended with respect to previous work of Ouahman et al. [Spectrochim. Acta 45A (1989) 175–179], to about thousand assigned transitions with $-6\le K·\Delta K\le +16$. Thanks to the completeness of the theoretical model, the global standard deviation of the reproduction of the experimental wavenumbers in the ν₂/ν₅ system was greatly improved with respect to the previous high-resolution study of Kwabia Tchana et al. [J. Mol. Spectrosc. 228 (2004) 441–452]. For this Coriolis-interacting band system, two different reduction schemes, according to the theory of Stříteská et al. [J. Mol. Spectrosc. 256 (2009) 135–140], were applied and were proved to be equally successful.