Rotational spectroscopy as a tool to investigate interactions between vibrational polyads in symmetric top molecules: Low-lying states v8 ≤ 2 of methyl cyanide, CH3CN

Rotational and rovibrational spectra of methyl cyanide were recorded to analyze interactions in low-lying vibrational states and to construct line lists for radio astronomical observations as well as for infrared spectroscopic investigations of planetary atmospheres. The rotational spectra cover large portions of the 36-1627 GHz region. In the infrared (IR), a spectrum was recorded for this study in the region of 2v(8) around 717 cm(-1) with assignments covering 684-765 cm-1. Additional spectra in the vs region were used to validate the analysis. Information on the K level structure of CH3CN is almost exclusively obtained from IR spectra, as are basics of the J level structure. The large amount and the high accuracy of the rotational data improves knowledge of the J level structure considerably. Moreover, since these data extend to much higher and K quantum numbers, they allowed us to investigate for the first time in depth local interactions between these states which occur at high K values. In particular, we have detected several interactions between v(8) = 1 and 2. Notably, there is a strong Delta v(8) = +/- 1, Delta K = 0, Delta l = +/- 3 Fermi resonance between v(8) = 1(-1) and v(8) = 2(+2) at K = 14. Pronounced effects in the spectrum are also caused by resonant Delta v(8) = +/- 1, Delta K = -/+ 2, Delta l = +/- 1 interactions between v(8) = 1 and 2 at K = 13, l = 1/K = 11, l = 0 and at K = 15, l = +1/K = 13, l = +2. An equivalent resonant interaction occurs between K = 14 of the ground vibrational state and K = 12, l = +1 of v(8) = 1 for which we present the first detailed account. A preliminary account was given in an earlier study on the ground vibrational state. Similar resonances were found for CH3CCH and, more recently, for CH3NC, warranting comparison of the results. From data pertaining to v(8) = 2, we also investigated rotational interactions with v(4) = 1 as well as Delta v(8) = +/- 1, Delta K = 0, Delta l = +/- 3 Fermi interactions between v(8) = 2 and 3. We have derived N-2- and self-broadening coefficients for the v(8), 2v(8) - v(8), and 2v(8) bands from previously determined v(4) values. Subsequently, we determined transition moments and intensities for the three IR bands. (C) 2015 Elsevier Inc. All rights reserved.