HIGH-RESOLUTION SPECTROSCOPY OF METHYL ISOCYANIDE - THE NU-4 FUNDAMENTAL AND GROUND-STATE CONSTANTS

The first completely resolved high-resolution infrared vibration-rotation spectrum of methyl isocyanide (CH3NC) has been obtained. The ν4 band was measured between 900 and 990 cm-1 with a resolution of 0.006 cm-1; 971 transitions were assigned to the P and R branches of the ν4 fundamental band spanning K = 0 to K = 9 and J = 0 to J = 65. Reliable and highly accurate sets of spectroscopic constants for both of the ν4 and ground states were generated. Small anomalies were found in a group of lines in the low-J region of the K = 4 subband and near J = 61 of the K = 3 subband. A perturbation analysis was performed based on symmetry considerations. A rotational resonance (Δl = ⊣ 1, Δk = ±2) between the ν4 and 3ν8±1 (kl < 0) was found to be the most probable origin of the anomalies, and the interaction matrix element has been derived. © 1992.