HIGH-RESOLUTION INFRARED-SPECTROSCOPY AND ANALYSIS OF THE METHYL-GROUP STRETCHING (NU(1)) OF CH3NC

The rotational structure of the symmetric C-H stretching fundamental band ν1 of methyl isocyanide has been revealed for the first time; 540 well-resolved spectral transitions were measured with a resolution of 0.004 cm-1 using an interferometric spectrometer and were assigned to ν1 lines with K = 0 to 9 and J up to 50. The ν1 levels are highly perturbed, exhibiting anomalous structure in most of the observed subbands. The origin of the stronger perturbations has been traced mainly to x, y-Coriolis interactions with the degenerate C-H stretch ν5 and with the combinations ν2 + 3ν∓18 and ν4 + ν∓16 + 2ν±28, to Fermi resonance with 2ν06, and to a higher-order anharmonic resonance with another combination state, presumably ν2 + 3ν-38. These interactions have been explicitly taken into account in a least-squares fit of the observed transitions, yielding spectroscopic constants for the ν1 state. Estimates of the spectroscopic constants for the perturbing states together with their coupling constants with ν1 are also reported. © 1993 Academic Press, Inc.