2023MNRAS.525.2154T

Two closely related isomeric pairs of cyanides, CH₃[CN/NC] and H₂C[CN/NC], are studied in cold, dark interstellar cloud conditions. In contrast to the diverse detections of methyl cyanide (CH₃CN) in space, methyl isocyanide (CH₃NC) has previously only been observed in warm and hot star-forming regions. We report the detection of CH₃NC in the cold pre-stellar core Taurus Molecular Cloud (TMC-1) using the Green Bank Telescope with a detection significance of 13.4σ. Hyperfine transitions in H₂CCN and quadrupole interactions in CH₃CN and CH₃NC were matched to a spectral line survey from the Green Bank Telescope Observations of TMC-1: Hunting for Aromatic Molecules large project on the Green Bank Telescope, resulting in abundances with respect to hydrogen of $1.92^{+0.13}_{-0.07} \times 10^{-9}$ for the cyanomethyl radical (H₂CCN), $5.02^{+3.08}_{-2.06} \times 10^{-10}$ for CH₃CN, and $2.97^{+2.10}_{-1.37} \times 10^{-11}$ for CH₃NC. Efforts to model these molecules with the three-phase gas-grain code NAUTILUS in TMC-1 conditions overproduce both CH₃CN and CH₃NC, though the ratio of ∼5.9 per cent is consistent across observations and models of these species. This may point to missing destruction routes in the model. The models capture the larger abundance of H₂CCN well. Dissociative recombination is found to be the primary production route for these molecules, and reactions with abundant ions are found to be the primary destruction routes. H + CH₃NC is investigated with transition state theory as a potential destruction route, but found to be too slow in cold cloud conditions to account for the discrepancy in modelled and observed abundances of CH₃NC.