IR Spectroscopy of Acid Insoluble Residues of Carbonaceous Chondrites

Abstract

Leger and Puget (1984) have suggested polycyclic aromatic hydrocarbons (PAHs) as being responsible for the unidentified infrared emission bands (UIRs). The idea that PAHs are a component of the interstellar medium originated with Donn (1968). Because of this interest, insoluble carbon of carbonaceous chondrites obtained for the purpose of 13C NMR spectroscopy (Cronin et al., 1987) appears to be due to its aromatic content an attractive material to investigate with infrared spectroscopy. This insoluble carbon is a residue of HF and HC1 dissolution of the meteorite. Sample prepared from the Orgueil (C1), Murchison (CM2), and Allende (CV3) carbonaceous chondrites were examined with FTIR spectroscopy in bulk form in KBr pellets, as films evaporated under vacuum onto KBr disks, and as the residues of those evaporations in KBr pellets. The 13C cross polarization, magic angle spinning sample nuclear magnetic resonance (CP-MASS NMR) data indicates two major peaks centered at 29 and 133 ppm relative to tetramethylsilane (Cronin et al., 1987). The strength of the 133 ppm peak due to 13C in aromatic structures, relative to the 29 ppm 13C peak from aliphatic constituents, increases in the sequence of Allende, Murchison, and Orgueil (Figure 1). The 13C aromatic peak of the Orgueil residue is of particular interest in that its lack of structure normally suggestive of carboxyl or methoxy groups, and narrowness indicative of a homogeneity of the aromatic species consistent with large PAH structures, is what would be expected of interstellar PAHs (Wdowiak 1986, Tielens et al., 1987). The 11.3 µm UIR band mandates a family of PAHs that have single hydrogens per peripheral ring as a major interstellar PAH constituent. Molecules such as coronene (Leger and Puget 1984) and hexabenzocoronene (Hendel et al., 1986) are precluded from being maior interstellar PAH species because they have two and three hydrogens per peripheral ring, and hence no 11,3 µm feature exists.