Interstellar Isomers: The Importance of Bonding Energy Differences

We present strong detections of methyl cyanide (CH₃CN), vinyl cyanide (CH₂CHCN), ethyl cyanide (CH₃CH₂CN), and cyanodiacetylene (HC₄CN) molecules with the Green Bank Telescope (GBT) toward the Sgr B2(N) molecular cloud. Attempts to detect the corresponding isocyanide isomers were only successful in the case of methyl isocyanide (CH₃NC) for its J_K = 1₀-0₀ transition, which is the first interstellar report of this line. To determine the spatial distribution of CH₃NC, we used archival Berkeley-Illinois-Maryland Association (BIMA) array data for the J_K = 4_K-3_K (K = 0-3) transitions, but no emission was detected. From ab initio calculations, the bonding energy difference between the cyanide and isocyanide molecules is >8500 cm^-1 (>12,000 K). Thus, cyanides are the more stable isomers and would likely be formed more preferentially over their isocyanide counterparts. That we detect CH₃NC emission with a single antenna (Gaussian beam size Ω_B = 1723 arcsec²) but not with an interferometer (Ω_B = 192 arcsec²) strongly suggests that CH₃NC has a widespread spatial distribution toward the Sgr B2(N) region. Other investigators have shown that CH₃CN is present both in the LMH hot core of Sgr B2(N) and in the surrounding medium, while we have shown that CH₃NC appears to be deficient in the LMH hot core. Thus, large-scale, nonthermal processes in the surrounding medium may account for the conversion of CH₃CN to CH₃NC, while the LMH hot core, which is dominated by thermal processes, does not produce a significant amount of CH₃NC. Ice analog experiments by other investigators have shown that radiation bombardment of CH₃CN can produce CH₃NC, thus supporting our observations. We conclude that isomers separated by such large bonding energy differences are distributed in different interstellar environments, making the evaluation of column density ratios between such isomers irrelevant unless it can be independently shown that these species are cospatial.