Toward a global model of the interactions in low-lying states of methyl cyanide: Rotational and rovibrational spectroscopy of the [formula omitted] state and tentative interstellar detection of the [formula omitted] 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 υ4=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 2ν8 around 717 cm−1, analyzed in our previous study, covered also the ν4 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 υ4=1 appears to be with υ8=3, ΔK=0,Δl=+3, a previously characterized anharmonic resonance. We report new analyses of interactions with ΔK=-2 and Δl=+1, with ΔK=-4 and Δl=-1, and with ΔK=-6 and Δl=-3; these four types of interactions connect all l substates of υ8=3 in energy to υ4=1. A known ΔK=-2,Δl=+1 interaction with υ7=1 was also analyzed, and investigations of the ΔK=+1, Δl=-2 and ΔK=+3,Δl=0 resonances with υ8=2 were improved, as were interactions between successive states with υ8⩽3, mainly through new υ8⩽2 rotational data.A preliminary single state analysis of the υ4=υ8=1 state was carried out based on rotational transition frequencies and on ν4+ν8-ν8 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 υ4=υ8=1.In addition to the interstellar detection of rotational transitions of methyl cyanide from within all vibrational states up to υ4=1, we report the tentative detection of υ4=υ8=1 toward the main hot molecular core of the protocluster Sagittarius B2(N) employing the Atacama Large Millimeter/submillimeter Array.