Knowledge Bank

Recently, interactions between vibrational polyads were studied for propyne, H$3$C$-$C$\equiv$CH; in particular those between $v10=1$ at 30,$\mu$m with the 15,$\mu$m dyad ($v9=1$, $v10=2$),} {\bf 216} (2002) 397$-$407.\}\}\; \{\backslash bf\; 102\}\; (2004)\; 1555$-$1568.\}\; as\; well\; as\; between\; that\; dyad\; and\; the\; 10\backslash ,$\mu$mtetrad($v_5 = 1$, $v9=v10=1$, $v10=3$, $v8=1$). Pronounced effects were caused by $\Delta v10=\pm 1$, $\Delta K=0$, $\Delta l=\pm 3$ 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$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$, respectively. The present investigations focus on the $v8=0$, 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 $\Delta v8=\pm 1$, $\Delta K=0$, $\Delta l=\pm 3$ Fermi-type resonances around $K$ of 14, additional fairly strong resonances occur at similar $K$ values which are described by $\Delta v8=\pm 1$, $\Delta K=\mp 2$, $\Delta l=\pm 1$. The latter type of resonance takes even place weakly between $v=0$ and $v8=1$; 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 $v8\le 2$ have been largely completed. The results will be compared with those in propyne.