Origin of organic matter in the early solar system—VII. The organic polymer in carbonaceous chondrites

Abstract The insoluble polymer from the Murchison C2 chondrite was studied by a variety of degradation techniques: pyrolysis, depolymerization by Na 4 P 2 O 7 or CF 3 COOH, and oxidation by HNO 3 , Na 2 Cr 2 O 7 , or O 2 /UV light. Products were identified by IR spectroscopy, gas chromatography, and mass spectrometry (time-of-flight and high-resolution). In some cases, parallel measurements were made on a synthetic polymer produced by the Fischer-Tropsch reaction, a meteoritic polymer from the Allende C3V chondrite, and samples of coal or related materials. Our studies confirm the prevailing view that the meteoritic polymer has a bridged aromatic structure with functional groups such as COOH, OH, and CO, but provides much new detail. Oxidation with HNO 3 shows that the meteoritic and synthetic polymers have a similar degree of condensation, greater than that of high-volatile bituminous coal. Gentler oxidation with Cr 2 O 2− 7 or O 2 /UV led to the identification of 15 aromatic ring systems as the corresponding carboxylic acids: benzene, biphenyl, naphthalene and phenanthrene and their methyl derivatives, fluoranthene (or pyrene), chrysene, fluorenone, benzophenone, anthraquinone; and the heterocyclics dibenzofuran, benzothiophene, dibenzothiophene, pyridine, quinoline (or isoquinoline), and carbazole. Of 11 aliphatic acids identified, three dicarboxylic acids presumably came from hydroaromatic portions of the polymer, whereas eight monocarboxylic acids probably are derived from bridging groups or ring substituents. Depolymerization with CF 3 COOH yielded some of the same ring systems, as well as alkanes (C 1 –C 8 ) and alkenes (C 2 –C 8 ), alkyl (C 1 –C 5 ) benzenes and naphthalenes, and methyl- or dimethyl-indene, -indane, -phenol, -pyrrole, and -pyridine. All these compounds were detected below 200°C, and hence probably were indigenous constituents rather than pyrolysis products. Though the match between the synthetic and meteoritic polymer is only fair, several properties of the latter suggest that it, too, was produced by surface catalysis: the predominance of n -alkyl fragments, its occurrence as a surface coating on specific kinds of mineral grains, and the C 13 C 12 fractionation between polymer and coexisting carbonates.