Detection of Ketenimine (CH ₂ CNH) in Sagittarius B2(N) Hot Cores

Ketenimine (CH2CNH) has been detected in absorption toward the star-forming region Sagittarius B2(N) with the 100 m Green Bank Telescope by means of three rotational transitions: 716-808 at 41.5 GHz, 819-909 at 23.2 GHz, and 918-100, 10 at 4.9 GHz. Ketenimine has a sparse rotational spectrum below 50 GHz. From transition line strength arguments, the spectral lines found are the ones most likely to be detected, and they occur in spectral regions that have little possibility of confusion with other molecular species. Partially resolved hyperfine structure is apparent in the 4.9 GHz transition, which has energy levels ~50 K above the ground-state level; the absorption seen in this transition appears to be emanating from gas in close proximity to the LMH hot core that has a systemic LSR velocity of +64 km s-1. By comparison, the 41.5 and 23.2 GHz transitions have lower energy levels of ~33 and ~41 K, respectively, and show absorption against the two star-forming Sgr B2(N) hot cores with systematic LSR velocities of +64 (the LMH) and +82 km s-1. These ketenimine data show that the hot core at +82 km s-1 is cooler than the hot core at +64 km s-1. Ketenimine is likely formed directly from its isomer methyl cyanide (CH3CN) by tautomerization driven by shocks that pervade the star-forming region.