High-Resolution Infrared Spectroscopy of Methyl Isocyanide: The ν3, ν6, and ν7 + ν8 Bands

Abstract

The vibration-rotation spectrum of methyl isocyanide (CH3NC) has been recorded with the aid of a high-resolution Fourier transform spectrometer in the region 1370 to 1560 cm−1 containing the perpendicular band of the fundamental vibration ν6 (species E), the weaker parallel band of the ν3 (A1) fundamental, and the perpendicular combination band ν∓7 + ν∓8 (E) enhanced by Fermi resonance with ν6. Sixteen hundred seventy well-resolved lines were assigned to 15 subbands of ν6, 6 subbands of ν3, and 3 subbands of ν−7 + ν−8. A strong x, y-Coriolis resonance between ν3 and ν6 and Fermi resonance between ν±6 and the E component ν∓7 + ν∓8, as well as between ν3 and the A1,2 components ν±7 + ν∓8, greatly affects the spectrum. Additional weaker anharmonic interaction of ν6 with the ν4 + 2ν28 combination and higher-order rotational interactions connecting the various states were also detected in the spectrum. All of these interactions have been incorporated into a 9 × 9 Hamiltonian matrix used for modeling the upper states of the observed transitions. A set of spectroscopic constants is reported for the upper states of the bands ν3, ν6, and ν7 + ν8 and for ν4 + 2ν28 which reproduces the observed lines with an overall standard deviation of 0.0012 cm−1.