Zhibin Wei

Synthesis of Higher Diamondoids and Implications for Their Formation in Petroleum

triamantane (3) and so forth, can be prepared by chemical synthesis. Of the higher diamondoids, i.e., those that have isomeric forms, only C2h-symmetric [121]tetramantane (4 a) has been prepared in the laboratory in very low yields. All other higher diamondoids are only accessible from raw petroleum. There are three tetramantanes (4a and [1(2)3]tetramantane, C3v-4b ) including one enantiomeric pair (P)-(+)and (M)-( )-[123]tetramantane (4c), six pentamantanes (with [1(2,3)4]pentamantane being the first exhibiting a diamond {111} surface), 24 hexamantanes (6), 12] nearly one hundred heptamantanes (7), and so forth. Thus far, diamondoids with up to 11 cages have been shown to exist in petroleum, but no other source is known, although recent studies suggest possible interstellar occurrence. The larger nanodiamonds occur as rigid rods (4a, 5c), discs (4b), 12] pyramids (5 a), and helices (4c, 5 f), exhibiting quantum confinement and negative electron affinity. They can be specifically derivatized, 11,17, 18] with electron emission properties superior to any other material making them attractive for molecular electronics. The mechanism for formation of these nanodiamonds for a long time was attributed to thermodynamically controlled carbocation rearrangements. 21] Such mechanisms enable the practical synthesis of 1–3 but they fail in the production of the higher diamondoids. 21, 22] A detailed analysis of the mechanism for adamantane formation from a single starting material shows an amazing 2897 pathways; a more limited analysis of triamantane formation through carbocation pathways indicates at least 300000 potential intermediates. Prospects for higher diamondoid syntheses by these pathways are bleak due to a lack of large polycyclic precursors, problems with intermediates trapped in local energy minima, disproportionation reactions leading to side products, and the exploding numbers of isomers as the size of target higher diamondoid products increases. With the failure of syntheses of higher diamondoids through carbocation rearrangements, attempts at their preparation were abandoned in the 1980s. Since higher diamondoids occur in relatively high concentrations in petroleum that has undergone thermal cracking (i.e., been subjected to very high temperatures due to deep burial), we began to consider that these free-radical cracking reactions might be involved in higher diamondoid formation. The uncatalyzed formation of 1 and 2 from n-alkanes under conditions of cracking was shown recently, presenting evidence that exclusively thermal pathways involving free radicals can readily compete with the typically assumed acidcatalyzed carbocation rearrangements. Such mechanistic proposals underline the notion that diamondoids are thermodynamically the most stable hydrocarbons, i.e., they are more stable than nanographenes (extended polycyclic aromatic hydrocarbons) of comparable molecular weight. Moreover, the relative stabilities of carbocations and alkyl radicals Scheme 1. The family of diamondoids: lower diamondoids 1–3, the three isomers of tetramantane (4), and the six pentamantanes (5). The numbers in brackets refer to the unique Balaban–Schleyer nomenclature.

The fate of diamondoids in coals and sedimentary rocks

Diamondoids were detected in the extracts of a series of coals and rocks varying in maturity, lithology, source input, and depositional environment. At the same maturity level, diamondoids are generally about a magnitude more abundant in source rocks than in coals. The concentrations of diamondoids are maturity dependent. However, while diamondoids become more abundant with the increasing thermal maturity, a diminution in diamondoid concentrations is observed at the maturity value of about R{sub o} = 4.0% in both coals and rocks. The occurrence of diamantane destruction at 550{sup o}C during pyrolysis suggests that diamondoids may be eventually destroyed at high temperatures in the Earth. Here we propose three main phases of diamondoid life in nature: diamondoid generation (phase I, R{sub o} 4.0%).