High-Resolution Infrared Spectroscopy of the ν4 + ν8 − ν8 Band of CH3NC

The vibration-rotation spectrum of the ν4 + ν8 − ν8 band for CH3NC has been observed and measured with an interferometric spectrometer with a spectral resolution of 0.004 cm−1; 175 transitions were assigned to the P( J, k, l) branches spanning J = 5-29 and k = 3-8. Spectroscopic constants for the ν4 + ν8 state were obtained through a least-squares fit by explicitly taking account of the l-resonance. The validity of the additivity rule of molecular constants is discussed. Small anomalies detected in the K = 3 and 4 subbands of the ν4 + ν8 − ν8 band were ascribed to a rotational resonance (Δl = ∓1, Δk = ±2) between the ν4 + ν8 and 4ν−28 states.