Diamond and Graphite Precursors Brium Concentration Is the More Stable

SCIENTIFIC CORRESPONDENCE

the two: the species with the higher equili Diamond and graphite precursors brium concentration is the more stable. Illustrative results of such calculations for SJR-Badziag et al. 1 have attempted to measures of stability for nucleation three diamond precursors and the cor explain the observation that diamond precursors, because it is essential to responding graphite precursors are shown is often a product in carbon-forming account for entropy loss during growth. in the table. They demonstrate that, even reactions. They find that, in the presence On this basis, I find that diamond pre under relatively mild pyrolysis conditions, of hydrogen, precursors to diamond cursors are always significantly less stable graphite precursors are far more stable nucleation are of lower energy than than graphite precursors under typical than diamond precursors with the same graphite precursors because of the influ conditions, so that there are greater number of carbon atoms. ence of surface C-H groups. Thus they thermodynamic impediments to diamond The 'ceiling temperatures' in the table conclude that homogeneous nucleation nucleation than to graphite nucleation. are temperatures above which the dia of diamond is preferred over graphite To compare the thermodynamic stabili mond precursors (cyclohexane, adaman nucleation. But equilibrium constants, ties of two substances, one must consider tane and diamantane) become less stable rather than energies, are the appropriate the balanced reaction that interconverts than the corresponding graphite precursor species in the presence of 1 atm H,. For EQUILIBRIA BETWEEN DIAMOND AND GRAPHITE PRECURSORS the three diamond precursors considered, this temperature falls between 500 K and Precursor L'l. ~298 L'l.S,,,gs TJ log (K,q):j: 550 K and shows little dependence on 1 1 Diamond Graphite (kcal mol- ) (cal moi-'K- ) (K) 700 K 1,000 K precursor size. The favourable enthalpies cyclohexane = benzene + 3 H, 49.3 90.1 547 4.3 8.9 of formation of the diamond precursors are overwhelmed by unfavourable entro adamantane = naphthalene+ 4 H, 67.6 130.6 517 7.3 13.8 pies of reaction above this temperature. diamantane = phenanthrene + 5 H, 88.5 164.9 536 8.4 16.7 Below this temperature, equilibrium Thermodynamic values from refs 4-7. Diamantane entropy is estimated from ring additivity calculations show that diamond precur 1 (86.5 cal moi-'K- ; ref. 8). sors are exceedingly unstable relative to t Temperature at which equilibrium constant is equal to unity (ceiling temperature). methane. Reactions under these condi :j: Keq is the equilibrium constant in atmospheric-pressure standard states. These equilibrium tions would lead to dissociation, not constants are equal to the relative concentrations of graphite and diamond precursors growth. when rH,] = 1 atm. These conclusions agree with our earlier work',] on hydrocarbon equilibria at high temperatures, which outlined the Child leukaemia curies or cars? thermodynamically preferred pathways SIR-Evans in News and Views 1 reviewed unsatisfactory exposure estimates for for carbon nucleation under such condi the evidence suggesting that the apparent cases and little is known about the shape tions. Growth involved polycyclic aromatic childhood leukaemia excess near the of the benzene/leukaemia dose-response hydrocarbons (graphite precursors); the Sellafield nuclear-fuel reprocessing plant curve'. Nevertheless, permitted occupa higher H/C ratios of other classes of in the United Kingdom is unlikely to be tional exposure levels in the United States compounds made them far less stable at linked to preconceptual paternal radi have recently been reduced from 10 to 1 high temperatures'. ation'. Evans finds particularly telling the parts per million'. Levels approaching this In view of the extreme instability of absence of increased risk of leukaemia in (780 parts per billion) have been recorded saturated hydrocarbons at high tempera offspring of Hiroshima/Nagasaki survi in cars in the United States, despite strict tures, a mechanism for diamond nuclea vors, the lack of increuse of inherited exhaust standards'. In California, ambient tion that involves these species seems congenital abnormalities near Sellafield air levels of only 4.6 parts per billion unlikely. A more plausible pathway (which would have been expected), as benzene have been suggested to con involves the initial formation of highly well as the presence of increased risk in tribute an excess lifetime risk of 101 to 780 energetic carbon species (C, C,, CH and some groups of workers outside the cases per million people exposed". so on) under severe conditions, followed nuclear industry. Evans considers that Such estimates are necessarily crude as by quenching and coalescence to form occupational exposure to chemical leukae they are based on questionable 'extra various carbonaceous solids. Other than mogens, such as benzene, or activation polation-through-zero' risk assumptions, the energy required to form such energetic of viral leukaemogens by "DNA but are derived from cases in occupation precursors, this pathway is not subject to damaging agents" are more plausible ally exposed adults. By contrast, little is major thermodynamic impediments. explanations. known about the impact of benzene (or STEPHEN E. STEIN I would like to suggest a further, related its maternal metabolites) in utero or in the Chemical Kinetics Division, hypothesis: that non-occupational pre context of naive postnatal metabolic National Institute of Standards and and postnatal exposure to benzene from detoxification systems. Technology, petrol evaporation and car exhausts may SIMON P. WOLFF Gaithersburg, be a significant contributory factor to Department of Clinical Pharmacology Maryland 20899, USA leukaemia risk. Sudden, local expansion University College London, 1. Badziag, P, Verwoerd, W.S., Ellis, W.P. & Greiner, N.R. in car ownership and usage caused by 5 University Street, Nature 343, 244-245 (1990). geographically isolated industrial and/or London WC1E 6JJ, UK 2. Stein, S.E. I phys. Chem. 82. 566-571 (1978). 3. Stein, S.E. & Fahr A. I phys. Chem. 89, 3714-3725 'new town' developments could result in (1985), the emergence and detection of leukaemia 1. Evans, J.H. Nature345, 16-17 (1990). 4. Stull, D.R., Westrum, E. & Sinke, G.W. The Chemical 'hot spots' as small but real statistical 2. Gardner, M.J. eta/. Br med. I 300, 423-429 (1990). Thermodynamics ofOrgamc Compounds (Wiley, New York, 3. Swaen, G.M. & Merjers, J.M. Br. I ind. Med. 46,826-830 1971) fluctuations above leukaemia incidence (1990). 5. Clark, T, Knox, T, McKervey, M.A., Mackie, H. & Rooney, baseline. 4. Nicholson, W.J & Landrigan, P.J Envir. Hlth Perspect. 82, J .J. I Am. Chem. Soc. 101, 2404--2410 (1979), 185-188 (1989). 6. Boyd, R.H, Sanwal, S.N., Shary-Tehrany, S. & McNally, D. There is considerable controversy 5. Gilks, J.M.L. eta/. CONCAWE: Oil Companies' European I phys. Chem. 75, 1264-1271 (1971). about the leukaemia risk associated even Organisation for Environmental and Health Protection. 7. Carson, A.S. et a/. I Chem. Thermodyn. 3, 915-918 Report no. 8/89 (1989). (1974). with occupational benzene exposure. Risk 6. Read, R.C. & Green, M. Br. med. I 300, 761-762 8. Stein, S.E. & Barton, B.D. Thermochim. Acta44. 265-281 estimates are derived from probably (1990). (1981).