Interstellar Isomers: The Importance of Bonding Energy Differences

We present strong detections of methyl cyanide, vinyl cyanide, ethyl cyanideand cyanodiacetylene molecules with the Green Bank Telescope (GBT) toward theSgr B2(N) molecular cloud. Attempts to detect the corresponding isocyanideisomers were only successful in the case of methyl isocyanide for itsJ(K)=1(0)-0(0) transition, which is the first interstellar report of this line.To determine the spatial distribution of methyl isocyanide, we used archivalBerkeley-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). That we detect methyl isocyanide emission with asingle antenna (Gaussian beamsize(Omega_B)=1723 arcsec^2) but not with aninterferometer (Omega_B=192 arcsec^2), strongly suggests that methyl isocyanidehas a widespread spatial distribution toward the Sgr B2(N) region. Thus,large-scale, non-thermal processes in the surrounding medium may account forthe conversion of methyl cyanide to methyl isocyanide while the LMH hot core,which is dominated by thermal processes, does not produce a significant amountof methyl isocyanide. Ice analog experiments by other investigators have shownthat radiation bombardment of methyl cyanide can produce methyl isocyanide,thus supporting our observations. We conclude that isomers separated by suchlarge bonding energy differences are distributed in different interstellarenvironments, making the evaluation of column density ratios between suchisomers irrelevant unless it can be independently shown that these species areco-spatial.Comment: 17 pages, 2 figures, accepted to the Astrophysical Journa