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)

Abstract

Rotational spectra of methyl cyanide were recorded newly and were analyzed together with existing spectra to extend the global model of low-lying vibrational states and their interactions to 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 2ν 8 around 717 cm −1 , analyzed in our previous study, covered also the ν 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 4 = 1 appears to be with 8 = 3, ∆K = 0, ∆l = +3, a previously characterized anharmonic resonance. We report new analyses of interactions with ∆K = −2 and ∆l = +1, with ∆K = −4 and ∆l = −1, and with ∆K = −6 and ∆l = −3; these four types of interactions connect all l substates of 8 = 3 in energy to 4 = 1. A known ∆K = −2, ∆l = +1 interaction with 7 = 1 was also analyzed, and investigations of the ∆K = +1, ∆l = −2 and ∆K = +3, ∆l = 0 resonances with 8 = 2 were improved, as were interactions between successive states with 8 ≤ 3, mainly through new 8 ≤ 2 rotational data. A preliminary single state analysis of the 4 = 8 = 1 state was carried out based on rotational transition frequencies and on ν 4 + ν 8 − ν 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 4 = 8 = 1.In addition to the interstellar detection of rotational transitions of methyl cyanide from within all vibrational states up to 4 = 1, we report the tentative detection of 4 = 8 = 1 toward the main hot molecular core of the protocluster Sagittarius B2(N) employing the Atacama Large Millimeter/submillimeter Array.

Laboratory spectroscopy for astrophysics: Festschrift for Stephan Schlemmer

“…Another contribution deals with the millimeter-wave spectrum of the 13 CH 3 OD isotopic species [81]. Extensive rotationvibration interactions in low-lying vibrational states of CH 3 CN were revealed [70]. Investigations of slightly larger molecules involve vinylacetylene [79], acetylacetylene [62], propanoic acid [87], ethyl methyl ether [64], methyl cyanoacetate [65], and propylene oxide [66].…”