Solid state CP/MAS C-13 NMR of the insoluble organic matter of the Orgueiland Murchison meteorites: quantitative study

Solid state CP/MAS C-13 nuclear magnetic resonance (NMR) study of the insol uble organic fraction isolated by HF/HCl treatment under an inert atmospher e from the Orgueil and Murchison carbonaceous meteorites is reported. Based on these spectra, eight different types of carbons (aro- and all-linked CH 3, CH2, aliphatic C-linked to heteroelements, protonated and non-protonated aromatic C, carboxyls and carbonyls) were identified, their relative amoun ts were determined and theoretical NMR-derived H/C atomic ratios were calcu lated. Comparison of these H/C ratios with those obtained from elemental an alysis revealed that a large part of the carbons of the insoluble fraction of both meteorites are not detected by CP/MAS C-13 NMR. Taking into account these undetected carbons, the relative abundance of aromatic carbons (as % of the total carbons) was calculated as between 69 and 78% in Orgueil and 61 and 67% in Murchison and thus shown to be much higher than previously th ought. In addition, the NMR data allowed to calculate the ratio of non-prot onated and protonated aromatic carbons. Based on previous studies on chemic al and thermal degradation, the present NMR data were interpreted as reflec ting the occurrence of highly substituted rather small aromatic moieties in these insoluble organic materials. Along with FTIR data, NMR results also pointed to a relatively high branching level in the aliphatic chains, espec ially for Murchison. According to the above results, the insoluble organic matter in these meteorites did not suffer a marked thermal maturation durin g the late hydrothermalism of the parent body. Its synthesis involved a sta tistical combination of all the possible bonds between -CH3, -CH2 and -CH r adicals, producing both the aliphatic and the aromatic moieties. This model explains not only the chemical features of the chondritic insoluble organi c material but also the typical FTIR signature of the interstellar medium. Therefore, organic material found in the interstellar medium and that incor porated in the most primitive objects of the solar system seem to share a c ommon organosynthesis. (C) 2000 Elsevier Science B.V. All rights reserved.