Toward a global model of the interactions in low-lying states of methyl cyanide: rotational and rovibrational spectroscopy of the v4 = 1 state and tentative interstellar detection of the v4 = v8 = 1 state in Sgr B2(N)

Rotational spectra of methyl cyanide were recorded newly and were analyzed together with existing spectra to extend the globalmodel of low-lying vibrational states and their interactions to 34 = 1 at 920 cm􀀀1. The rotational spectra cover large portions ofthe 36􀀀1439 GHz region and reach quantum numbers J and K of 79 and 16, respectively. Information on the K level structure ofCH3CN is obtained from IR spectra. A spectrum of 28 around 717 cm􀀀1, analyzed in our previous study, covered also the 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 34 = 1 appears to be with 38 = 3, K = 0, l = +3, a previously characterized anharmonicresonance. We report new analyses of interactions with K = 􀀀2 and l = +1, with K = 􀀀4 and l = 􀀀1, and with K = 􀀀6and l = 􀀀3; these four types of interactions connect all l substates of 38 = 3 in energy to 34 = 1. A known K = 􀀀2, l = +1interaction with 37 = 1 was also analyzed, and investigations of the K = +1, l = 􀀀2 and K = +3, l = 0 resonances with38 = 2 were improved, as were interactions between successive states with 38 3, mainly through new 38 2 rotational data.A preliminary single state analysis of the 34 = 38 = 1 state was carried out based on rotational transition frequencies and on4 + 8 􀀀 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 34 = 38 = 1.In addition to the interstellar detection of rotational transitions of methyl cyanide from within all vibrational states up to 34 = 1,we report the tentative detection of 34 = 38 = 1 toward the main hot molecular core of the protocluster Sagittarius B2(N) employingthe Atacama Large Millimeter/submillimeter Array.