CH_3_NCO (methyl isocyanate) transition frequencies

CH_3_NCO was identified in spectra covering 48GHz (68-116GHz) in the 3mm segment of a broadband survey of Sgr B2(N). Thirty very favorable rotational lines (K_a_=0 and K_a_=1 only; E_u_<60K) originating in five consecutive transitions (J=8->7, 9->8, 10->9, 11->10, and 12->11) in both the A and E internal rotation species are present in this frequency range.

Emission was observed at all of the predicted frequencies, with 17 lines appearing as distinct, uncontaminated spectral features, clearly showing the classic a-type, asymmetric top pattern, with T_R_^*^~20-70mK. The CH_3_NCO spectra also appear to exhibit two velocity components near V_LSR_~62 and 73km/s, both with {Delta}V_1/2_~10km/s --typical of molecules such as CH_2_CHCN, HNCO, and HCOOCH_3_ in Sgr B2(N). The column density of CH_3_NCO in Sgr B2(N) was determined to be N_tot_~2.3x10^13^ and 1.5x10^13^/cm^2^ for the 62 and 73km/s components, corresponding to fractional abundances, relative to H_2_, of f~7.6x10^-12^ and 5.0x10^-12^, respectively.