Toward a global model of the interactions in low-lying states of methyl cyanide: rotational and rovibrational spectroscopy of the υ4=1 state and tentative interstellar detection of the υ4=υ8=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 υ 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 CH3CN is obtained from IR spectra. A spectrum of 2 ν 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 υ 4 = 1 appears to be with υ 8 = 3 , Δ K = 0 , Δ l = + 3 , a previously characterized anharmonic resonance. We report new analyses of interactions with Δ K = - 2 and Δ l = + 1 , with Δ K = - 4 and Δ l = - 1 , and with Δ K = - 6 and Δ l = - 3 ; these four types of interactions connect all l substates of υ 8 = 3 in energy to υ 4 = 1 . A known Δ K = - 2 , Δ l = + 1 interaction with υ 7 = 1 was also analyzed, and investigations of the Δ K = + 1 , Δ l = - 2 and Δ K = + 3 , Δ l = 0 resonances with υ 8 = 2 were improved, as were interactions between successive states with υ 8 ⩽ 3 , mainly through new υ 8 ⩽ 2 rotational data. A preliminary single state analysis of the υ 4 = υ 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 υ 4 = υ 8 = 1 . In addition to the interstellar detection of rotational transitions of methyl cyanide from within all vibrational states up to υ 4 = 1 , we report the tentative detection of υ 4 = υ 8 = 1 toward the main hot molecular core of the protocluster Sagittarius B2(N) employing the Atacama Large Millimeter/submillimeter Array.