Molecular beam infrared spectrum of nitromethane in the region of the first C-H stretching overtone

The infrared spectrum of the first C-H stretching overtone of nitromethane (CH3NO2) was measured in the wavenumber region 6047-6055 cm(-1) with a resolution of 5 MHz using a molecular beam laser spectrometer equipped with bolometric optothermal detection and a power build-up cavity. The assignment of the spectrum was made using ground-state combination differences and an asymmetric rotor energy level simulation program. The following parameters were obtained for the m = 0 upper state: nu(0) = 6050.1378(24) cm(-1), A = 0.55399(84) cm(-1), B = 0.2936(15) cm(-1), C = 0.2567(13) cm(-1). The intramolecular vibrational energy redistribution (IVR) rate of the C-H stretch was calculated from the spectrum. The IVR rate of the different J(KaKc) levels in the spectrum ranged from 170 to 300 ps, comparable to the rate measured for methyl C-H stretching overtones in molecules of similar size (CH3SiH3 and CH3CD3).