ROTATIONAL SPECTROSCOPY AS A TOOL TO INVESTIGATE INTERACTIONS BETWEEN VIBRATIONAL POLYADS IN SYMMETRIC TOP MOLECULES: LOW-LYING STATES OF METHYL CYANIDE
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Date
2007
Journal Title
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Publisher
Ohio State University
Abstract
Recently, interactions between vibrational polyads were studied for propyne, H$_3$C$-$C$\equiv$CH; in particular those between $v_{10} = 1$ at 30\,$\mu$m with the 15\,$\mu$m dyad ($v_9 = 1$, $v_{10} = 2$),} {\bf 216} (2002) 397$-$407.}} {\bf 102} (2004) 1555$-$1568.} as well as between that dyad and the 10\,$\mu$m tetrad ($v_5 = 1$, $v_9 = v_{10} = 1$, $v_{10} = 3$, $v_8 = 1$)$^b$. Pronounced effects were caused by $\Delta v_{10} = \pm 1$, $\Delta K = 0$, $\Delta l = \pm 3$ Fermi-type resonances at $K \approx 12$. Such resonances had not been found thus far for the isoelectronic methyl cyanide, H$_3$C$-$C$\equiv$N, molecule despite extensive previous spectroscopic work. As methyl cyanide is also an important interstellar molecule, in particular in hot and dense molecular cores, and as it may play a role in the atmospheres of planets or of Titan, we have recorded extensive rotational and rovibrational spectra up to $\sim 1.6$\,THz and $\sim 1500$\,cm$^{-1}$, respectively. The present investigations focus on the $v_8 = 0$, 1, and 2 states. The $\nu_8$ mode in methyl cyanide corresponds to the $\nu_{10}$ mode in propyne, and it is at a rather similar energy. While the infrared data pertaining to these states help to constrain their $K$ level structures they do not reach $K \approx 14$ which are perturbed most; the $l = 0$ component of 2\,$\nu_8$ may be an exception. The pure rotational data on the other hand access $K$ levels well beyond these perturbations which can be easily recognized in the spectra. Since the $\nu_9 = \delta$(CCH) mode in propyne is missing in methyl cyanide one would expect easier assignments and analyses. However, besides $\Delta v_8 = \pm 1$, $\Delta K = 0$, $\Delta l = \pm 3$ Fermi-type resonances around $K$ of 14, additional fairly strong resonances occur at similar $K$ values which are described by $\Delta v_8 = \pm 1$, $\Delta K = \mp 2$, $\Delta l = \pm 1$. The latter type of resonance takes even place weakly between $v = 0$ and $v_8 = 1$; an indication for this was seen previously.}ime\v{c}kov{a} {\it et al.}, {\it J. Mol. Spectrosc.} (2004) 123$-$126.} The analyses of interactions between states with $v_8 \leq 2$ have been largely completed. The results will be compared with those in propyne.
Description
Author Institution: I. Physikalisches Institut, Univ. Koln, 50937 Koln; and MPIfR, 53121 Bonn, Germany; JPL, California Inst. of Technol., Pasadena, CA 91109, USA; LISA, Univ. Paris 12 \& Paris 7 \& CNRS, 94010 Cr{e}teil, France; PNNL, Richland, WA 99352, USA