Radical pathways in the thermal decomposition of pyridine and diazines: a laser pyrolysis and semi-empirical study

Abstract

The mechanisms of thermal decomposition of pyridine and the three isomeric diazines have been investigated by IR laser pyrolysis in conjunction with stable end-product analysis by FTIR, NMR and GC–MS, and radical detection by EPR spectroscopy. Calculations at semi-empirical and ab initio levels have provided confirmation of potential reaction pathways. For pyridine, reaction is initiated by formation of pyridyl radicals, as indicated by extensive isotope exchange with added deuterium. Experiments with bromopyridines show that open chain radicals arising from ring opening of 2-pyridyl and 3-pyridyl radicals each lead to stable gaseous products, while 4-pyridyl radicals produce solid deposits, and may be the principal agents in soot formation. The evidence suggests that 1,2-diazine decomposes via a molecular route leading to stoichiometric production of HCN and C₂H₂, while 1,3- and 1,4-diazine follow a pattern of H loss and ring radical opening analogous to that of pyridine.

References

1. D. W. Pershing and J. O. L. Wendt, Sixteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, PA, 1977, p. 389.
2. P. C. Painter and M. M. Coleman, Fuel, 1979, 58, 301.
3. J. Jones, G. B. Bacskay, J. C. Mackie and A. Doughty, J. Chem. Soc., Faraday Trans., 1995, 91, 1587.
4. C. F. Roth, Ber., 1886, 19, 360.
5. H. Meyer and A. Hoffmann-Meyer, J. Prakt. Chem., 1921, 102, 287.
6. S. Ruhemann, Braunkohle, 1929, 28, 749.
7. C. D. Hurd and J. I. Simon, J. Am. Chem. Soc., 1962, 84, 4519.
8. A. E. Axworthy, V. H. Dayan and G. B. Martin, Fuel, 1978, 57, 29.
9. T. J. Houser, M. E. McCarville and T. Biftu, Int. J. Chem. Kinet., 1980, 12, 555.
10. R. D. Kern, J. N. Yong, J. H. Kiefer and J. N. Shah, 16th International Symposium on Shock Tubes and Waves, VCH, Weinheim, 1987, p. 437.
11. H. I. Leidreiter and H. Gg. Wagner, Z. Phys. Chem., 1987, 153, 99.
12. J. C. Mackie, M. B. Colket and P. F. Nelson, J. Phys. Chem., 1990, 94, 4099.
13. E. Ikeda and J. C. Mackie, J. Anal. App. Pyrolys., 1995, 34, 47.
14. A. Doughty and J. C. Mackie, J. Chem. Soc., Faraday Trans., 1994, 90, 541.
15. A. Doughty, J. C. Mackie and J. M. Palmer, Twenty-Fifth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, PA, 1994, p. 893.
16. H. Hettema, N. R. Hore, N. D. Renner and D. K. Russell, Aust. J. Chem., 1997, 50, 363.
17. J. Buckingham and F. Macdonald, Dictionary of Organic Compounds, 6th edn., Chapman & Hall, London, 1996.
18. W. M. Shaub and S. H. Bauer, Int. J. Chem. Kinet., 1975, 7, 509.
19. J. Pola, Collect. Czech. Chem. Commun., 1981, 46, 2856.
20. D. K. Russell, Chem. Soc. Rev., 1990, 19, 407; Coord. Chem. Rev., 1992, 112, 131; Chem. Vap. Deposition, 1996, 2, 223, and references therein.
21. MOLSPEC for Windows, 1993, Laser Photonics Inc., Andover, MA.
22. L. S. Rothman, R. R. Gamache, A. Goldman, L. R. Brown, R. A. Toth, H. M. Pickett, R. L. Poynter, J.-M. Flaud, C. Camy-Peyret, A. Barbe, N. Husson, C. P. Rinsland and M. A. H. Smith, App. Optics, 1986, 26, 4058.
23. N. J. Bristow, B. D. Moore, M. Poliakoff, G. S. Ryott and J. J. Turner, J. Organomet. Chem., 1984, 260, 180.
24. J. J. P. Stewart, J. Comp.-Aided. Mol. Design, 1990, 4, 1.
25. PC Spartan, Wavefunction Inc., Irvine, CA, 1996.
26. M. H. Brodsky and R. S. Title, Phys. Rev. Lett., 1969, 23, 581.
27. G. R. Allen and D. K. Russell, unpublished work.
28. J. Phys. & Chem. Ref. Data, 1982, 11, Supplement 2.
29. S. W. Benson, F. R. Cruikshank, D. M. Golden, G. R. Haugen, H. E. O'Neal, A. S. Rodgers, R. Shaw and R. Walsh, Chem. Rev., 1969, 69, 279.
30. J. R. Brown, N. R. Hore, D. K. Russell and P. A. Schwerdtfeger, unpublished results.