Toward a Global Model of Low-Lying Vibrational States of CH3CN: Overview and Interactions in the ν4 = 1 State at 920 cm^-1 with Nearby States

Methyl cyanide, CH3CN, is an important interstellar molecule, in particular in hot and dense molecular cores, and it may play a role in the atmospheres of planets or of Titan. Therefore, we have recorded extensive rotational and rovibrational spectra up to ~ 1:6 THz and ~ 1500 cm−1, respectively. The present investigation extends our analysis of states with v8 ≤ 2 at vibrational energies below 740 cm−1,1 which will be summarized briefly, and takes into consideration findings from an analysis of the ν4 band and the higher-lying ν7 (at ~1042 cm−1) and 3ν1 8 (at ~1078 cm−1) bands.2 The rotational data extend to J = 87 and K = 15, infrared assignments currently extend to 55 and 12, respectively. Parameters affecting only v7 = 1 or v8 = 3 as well as some additional interaction parameters were kept fixed to values from (b). The largest perturbations of v4 = 1 are caused by a Δk = 0, Δl = 3 interaction with v8 = 3 at K = 8. Despite the inclusion of the interaction parameter and a centrifugal distortion correction, residuals amount to more than 200 MHz very close to the resonance. Removal of these residuals probably requires explicit inclusion of v8 = 3 data. Several additional perturbations exist at lower as well as higher K with v8 = 2, v7 = 1 and v8 = 3. Higher values of K are difficult to reproduce in spite of an extensive set of distortion parameters which, at highest orders, have rather large magnitudes, possibly indicating unaccounted interactions which would probably occur with states even higher than v8 = 3.