Toward a global model of the interactions in low-lying states of methyl cyanide: rotational and rovibrational spectroscopy of the $v_4 = 1$ state and tentative interstellar detection of the $v_4 = v_8 = 1$ state in Sgr B2(N)

TL;DR

This study extends the global model of methyl cyanide's low-lying vibrational states by analyzing extensive rotational spectra, revealing complex interactions and tentatively detecting vibrational states in interstellar space.

Contribution

It provides new analyses of vibrational interactions in methyl cyanide and reports the first tentative interstellar detection of the $v_4=v_8=1$ state.

Findings

Extended the spectral model to include $v_4=1$ state.

Identified complex vibrational interactions and resonances.

Tentative detection of $v_4=v_8=1$ state in Sgr B2(N).

Abstract

New and existing rotational spectra of methyl cyanide were analyzed to extend the global model of low-lying vibrational states and their interactions to $v_4=1$ at 920 cm$^{-1}$. The rotational spectra cover large portions of the 36$-$1439 GHz region and reach quantum numbers $J$ and $K$ of 79 and 16, respectively. Information on the $K$ level structure of CH$_3$CN is obtained from IR spectra. A spectrum of $2\nu_8$ around 717 cm$^{-1}$, analyzed in our previous study, covered also the $\nu_4$ band. The assignments in this band cover 880$-$952 cm$^{-1}$, attaining quantum numbers $J$ and $K$ of 61 and 13, respectively. The most important interaction of $v_4=1$ appears to be with $v_8=3$, $\Delta K=0$, $\Delta l=+3$, a previously characterized anharmonic resonance. We report new analyses of interactions with $\Delta K=-2$ and $\Delta l=+1$, with $\Delta K=-4$ and $\Delta l=-1$, and with $\Delta K=-6$ and $\Delta l=-3$; these four types of interactions connect all $l$ substates of $v_8=3$ in energy to $v_4=1$. A known $\Delta K=-2$, $\Delta l=+1$ interaction with $v_7=1$ was also analyzed, and investigations of the $\Delta K=+1$, $\Delta l=-2$ and $\Delta K=+3$, $\Delta l=0$ resonances with $v_8=2$ were improved, as were interactions between successive states with $v_8\le 3$, mainly through new $v_8\le 2$ rotational data. A preliminary single state analysis of the $v_4=v_8=1$ state was carried out based on rotational transition frequencies and on $\nu_4+\nu_8-\nu_8$ hot band data. A considerable fraction of the $K$ levels was reproduced within uncertainties in its entirety or in part, despite obvious widespread perturbations in $v_4=v_8=1$. We detect rotational transitions of methyl cyanide from within all vibrational states up to $v_4=1$ and $v_4=v_8=1$ tentatively toward the hot molecular core of Sagittarius B2(N) employing the Atacama Large Millimeter Array.