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: ν0 ) 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 JKaKc 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).