The Organic Refractory Material in the Diffuse Interstellar Medium: Mid‐Infrared Spectroscopic Constraints

Abstract

This is an analysis of the 4000È1000 cm~1 (2.5È10 km) region of the spectrum of di †use interstellar medium (DISM) dust compared with the spectra of 13 materials produced in the laboratory which serve as analogs to the interstellar material. The organic signatures of extragalactic dust, carbonaceous chondritic material, and E. coli bacteria are also presented because these have been discussed in the literature as relevant to the di †use interstellar medium. Spectral analysis of the DISM allows us to place signiÐ-cant constraints on the applicability of proposed candidate materials. The spectra of candidate materials are evaluated using four spectral characteristics based on the interstellar data : (i) comparisons of the proÐle and subpeak positions of the 2940 cm~1 (3.4 km) aliphatic CH stretching-mode band, (ii) the ratio of the optical depth (O.D.) of the aliphatic CH stretch to the O.D. of the OH stretch near 3200 cm~1 (3.1 km), (iii) the ratio of the O.D. of the aliphatic CH stretch to the O.D. of the carbonyl band near 1700 cm~1 (5.9 km), and (iv) the ratio of the O.D. of the aliphatic CH stretch feature to the O.D. of the CH deformation modes near 1470 cm~1 (6.8 km) and 1370 cm~1 (7.25 km). We conclude that the organic refractory material in the di †use interstellar medium is predominantly hydrocarbon in nature, possessing little nitrogen or oxygen, with the carbon distributed between the aromatic and aliphatic forms. Long alkane chains with n much greater than 4 or 5 are not major constituents H 3 Cw(CH 2 ) n w of this material. Comparisons to laboratory analogs indicate the DISM organic material resembles plasma processed pure hydrocarbon residues much more so than energetically processed ice residues. This result is consistent with a birth site for the carrier of the 3.4 km band in the outÑow region of evolved carbon stars. The organic material extracted from the Murchison carbonaceous meteorite and the spectrum of E. coli bacteria reveal spectral features in the 5È10 km region that are absent in the DISM. Although the presence of unaltered circumstellar components in the Murchison meteorite has been established through several lines of evidence, it is unclear whether or not the aliphatic component which gives rise to the 3.4 km band is in that category. Considering the complete 2È10 km wavelength region, there is no spectral evidence for a biological origin of the 3.4 km interstellar absorption band. The similarity of the aliphatic CH stretch region of dust from our own Galaxy compared with that of distant galaxies suggests that the organic component of the ISM is widespread and may be an important universal reservoir of prebiotic organic carbon.

Crossed Molecular Beam Study on the Formation of Phenylacetylene and Its Relevance to Titan’s Atmosphere

“…It should also be noted that the primary reactant beam also contains carbon atoms as well as dicarbon and tricarbon molecules. Previous studies showed that tricarbon molecules have a significant entrance barrier upon reacting with benzene of more than 90 kJ mol -1 ; 57 therefore, under our experimental conditions, tricarbon does not react with benzene-d 6 . The lighter carbon (12 amu) and dicarbon reactants (24 amu) react with benzene-d 6 ; 58,59 due to the heavier ethynyl-d 1 radical (26 amu), carbon and dicarbon reactions lead only to products, which are lower in mass by 2 and 14 amu compared to those formed in the reaction of ethynyl-d 1 radicals with benzene-d 6 .…”

“…This experiment was compared with a novel electronic structure calculation on the singlet C 8 D 7 surface. Phenylacetylene-d 6 is the primary product formed via indirect reactive scattering dynamics through a longlived reaction intermediate initiated by a barrierless addition of the ethynyl-d 1 radical to benzene-d 6 . This initial intermediate was found to decompose via a tight exit transition state located about 42 kJ mol -1 above the separated products; here, the deuterium atom is ejected almost perpendicularly to the rotational plane of the decomposing intermediate and almost parallel to the total angular momentum vector.…”

“…Note that previous electronic structure calculations 37,38 predicted multiple thermodynamically stable C 8 H 6 isomers such as cyclic C 8 H 6 . However, among all the possible isomers synthesized, the only molecule formed in the ethynyl-d 1 plus benzene-d 6 reaction via an exoergic process was the phenylaceteylene-d 6 species. Besides the ethynyl-d 1 versus deuterium exchange pathway to form the phenylacetylene-d 6 molecule, the only other exoergic channel was the deuterium abstraction to form acetylened 2 plus the phenyl-d 5 radical.…”

“…This is clearly above our collision energy and also above the equivalent temperature in Titan's atmosphere. Thus, the abstraction reaction pathway does not compete with the formation of phenylacetylene-d 6 . To summarize, on the basis of the energetics of the reaction and the comparison with NIST data and electronic structure calculations, the phenylacetylene molecule along with atomic deuterium are concluded to be the primary products.…”

“…These findings and the suggested reaction mechanism gain full support from the computed potential energy surfaces ( Figure 7) adapted from Landera et al 37 In this figure, only the pathways leading to the formation of phenylacetylene are shown. It should be stressed that our reaction was conducted with ethynyl-d 1 and benzened 6 . Consequently, the pathways relevant to the reaction dynamics on the formation of phenylacetylene have been recalculated to account for the difference in the zero point energy.…”