Interstellar Isomers: The Importance of Bonding Energy Differences

Abstract

We present strong detections of methyl cyanide (CH 3 CN), vinyl cyanide (CH 2 CHCN), ethyl cyanide (CH 3 CH 2 CN), and cyanodiacetylene (HC 4 CN) 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 (CH 3 NC ) for its J K ¼ 1 0 0 0 transition, which is the first interstellar report of this line. To determine the spatial distribution of CH 3 NC, we used archival Berkeley-Illinois-Maryland Association ( BIMA) array data for the J K ¼ 4 K 3 K (K ¼ 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 À1 (>12,000 K ). Thus, cyanides are the more stable isomers and would likely be formed more preferentially over their isocyanide counterparts. That we detect CH 3 NC emission with a single antenna (Gaussian beam size B ¼ 1723 arcsec 2 ) but not with an interferometer ( B ¼ 192 arcsec 2 ) strongly suggests that CH 3 NC has a widespread spatial distribution toward the Sgr B2( N ) region. Other investigators have shown that CH 3 CN is present both in the LMH hot core of Sgr B2( N ) and in the surrounding medium, while we have shown that CH 3 NC appears to be deficient in the LMH hot core. Thus, largescale, nonthermal processes in the surrounding medium may account for the conversion of CH 3 CN to CH 3 NC, while the LMH hot core, which is dominated by thermal processes, does not produce a significant amount of CH 3 NC. Ice analog experiments by other investigators have shown that radiation bombardment of CH 3 CN can produce CH 3 NC, 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.

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

“…When the CH 3 CN ice was irradiated with UV, only CH 3 NC was produced. Remijan et al (2005) cited these results and other studies as supporting their suggestion that isocyanides were not formed primarily in thermal processes.…”

“…Currently, among interstellar molecules, there are 13 isomer pairs and three isomer triads, accounting for the statistic that nearly 40% of all interstellar molecules that could have isomeric counterparts do, in fact, have them. Moreover, Remijan et al (2005) examined observational results toward Sgr B2(N-LMH) for a number of cyanide and isocyanide isomers with respect to bonding energy differences and its effect on whether or not a particular isomer in a set might be formed in sufficient abundance to be detected with current radio telescopes. Furthermore, Lovas et al (2006) reported studies of the three isomers methylcyanoacetylene (CH 3 CCCN), cyanoallene (CH 2 CCHCN), and 3-butynenitrile (HCCCH 2 CN) toward TMC-1 and found that only the isomer with the least bonding energy (shallowest potential well), HCCCH 2 CN, was not readily observed.…”

The IRAM-30 m line survey of the Horsehead PDR

“…This is the first clear detection of this molecule in the millimeter domain. A detection of the J = 1−0 line of CH 3 NC at centimeter wavelengths (i.e., 20.1 GHz) has been reported by Irvine & Schloerb (1984) in TMC-1 and Remijan et al (2005) in SgrB2. At millimeter wavelengths, Cernicharo et al (1988) reported a tentative detection of the J = 4−3, J = 5−4 and J = 7−6 lines in SgrB2 but the large linewidths (20 km s −1 FWHM) and contamination from numerous other lines prevented a robust identification.…”

Quantum chemical study of a new reaction pathway for the adenine formation in the interstellar space

“…HCCN is found to be present in cold (Herbst & Millar 2007) and hot interstellar clouds. A radical with unusual allenic structure, it is found to be about twice as abundant as the stable species CH 3 CN (Remijan et al 2005) in the circumstellar envelope IRC+10216, but much less in the giant clouds Sgr B2 and, Orion. In the cold interstellar clouds, its fractional abundance is expected to be about 2.78 × 10 −11 with regard to H 2 .…”