Neutral–neutral reactions at the temperatures of interstellar clouds Rate coefficients for reactions of C₂H radicals with O₂, C₂H₂, C₂H₄ and C₃H₆ down to 15 K

Abstract

A CRESU (Cine′tique de Re′action en Ecoulement Supersonique Uniforme) apparatus has been used to measure rate constants for the reactions of the ethynyl radical (C₂H) with O₂, C₂H₂, C₂H₄ and C₃H₆ at temperatures from 295 down to 15 K. C₂H radicals are generated by photolysis of C₂H₂ at 193 nm using an ArF excimer laser and reaction rates are determined by observing the chemiluminescence from CH(A²Δ) which is generated in a minor channel of the reaction between C₂H radicals and O₂. The rate constants for all four reactions increase as the temperature is lowered, and those for reactions with the unsaturated hydrocarbons exceed 10^-10 cm³ molecule^-1 s^-1 at all temperatures below 100 K. The results confirm that C₂H radicals, like CN radicals, react rapidly with unsaturated hydrocarbons at very low temperatures and, in both cases, the radical replaces an H atom in the unsaturated molecule. It therefore seems likely that combinations of these reactions play a major role in forming species with long C chains and in synthesising the cyanopolyynes in dense interstellar clouds.

References

1. (a) I. R. Sims, J.-L. Queffelec, A. Defrance, C. Rebrion-Rowe, D. Travers, B. R. Rowe and I. W. M. Smith, J. Chem. Phys., 1992, 97, 8798; (b) I. R. Sims, J.-L. Queffelec, A. Defrance, C. Rebrion-Rowe, D. Travers, P. Bocherel, B. R. Rowe and I. W. M. Smith, J. Chem. Phys., 1994, 100, 4229.
2. I. R. Sims, J.-L. Queffelec, A. Defrance, D. Travers, B. R. Rowe, L. Herbert, J. Karthäuser and I. W. M. Smith, Chem. Phys. Lett., 1993, 211, 461.
3. I. R. Sims, P. Bocherel, A. Defrance, D. Travers, B. R. Rowe and I. W. M. Smith, J. Chem. Soc., Faraday Trans., 1994, 90, 1473.
4. I. R. Sims, I. W. M. Smith, D. C. Clary, P. Bocherel and B. R. Rowe, J. Chem. Phys., 1994, 101, 1748.
5. P. Sharkey, I. R. Sims, I. W. M. Smith, P. Bocherel and B. R. Rowe, J. Chem. Soc., Faraday Trans., 1994, 90, 3609.
6. I. R. Sims and I. W. M. Smith, Annu. Rev. Phys. Chem., 1995, 46, 109.
7. P. Bocherel, L. B. Herbert, B. R. Rowe, I. R. Sims, I. W. M. Smith and D. Travers, J. Phys. Chem., 1996, 100, 3063.
8. L. Herbert, I. R. Sims, I. W. M. Smith, D. W. A. Stewart, A. Canosa and B. R. Rowe, J. Phys. Chem., 1996, 100, 14928.
9. R. A. Brownsword, A. Canosa, B. R. Rowe, I. R. Sims, I. W. M. Smith, D. W. A. Stewart, A. C. Symonds and D. Travers, J. Chem. Phys., 1997, 106, 7662.
10. I. W. M. Smith, IAU Symposium 178: Molecules in Astrophysics, Probes and Processes, Kluwer, Dordrecht, 1997, p. 253.
11. A. Canosa, I. R. Sims, D. Travers, I. W. M. Smith and B. R. Rowe, Astron. Astrophys., 1997, 323, 644.
12. S. O. Le Picard, A. Canosa, D. Travers, D. Chastaing, B. R. Rowe and T. Stoecklin, J. Phys. Chem. A, 1997, 101, 9988.
13. P. L. James, I. R. Sims and I. W. M. Smith, Chem. Phys. Lett., 1997, 272, 412.
14. P. L. James, I. R. Sims and I. W. M. Smith, Chem. Phys. Lett., 1997, 276, 423.
15. P. L. James, I. R. Sims, I. W. M. Smith, M. H. Alexander and M. Yang, J. Chem. Phys., 1998, in press.
16. E. Herbst, H.-H. Lee, D. A. Howe and T. J. Millar, Mon. Not. R. Astronom. Soc., 1994, 268, 335.
17. R. P. A. Bettens, H.-H. Lee and E. Herbst, Astrophys. J., 1995, 443, 664.
18. E. Herbst, Annu. Rev. Phys. Chem., 1995, 46, 27.
19. M. B. Bell, P. A. Feldman, M. J. Travers, M. C. McCarthy, C. A. Gottleib and P. Thaddeus, Astrophys. J., 1997, 483, L61.
20. G. Winnewisser and E. Herbst, Top. Curr. Chem., 1987, 139, 119.
21. D. K. Bohme, S. Wlodek and A. B. Raksit, Can. J. Chem., 1987, 65, 2057.
22. E. Herbst and C. M. Leung, Astrophys. J. Suppl., 1989, 69, 271.
23. I. W. M. Smith and I. R. Sims, J. Chem. Soc., Faraday Trans., 1993, 89, 2166.
24. N. Winstanley and L. A. M. Nejad, Astrophys. Space Sci., 1996, 240, 13.
25. I. Cherchneff, A. E. Glassgold and G. A. Mamon, Astrophys. J., 1993, 410, 188.
26. (a) A. M. Renlund, F. Shokoohi, H. Reisler and C. Wittig, Chem. Phys. Lett., 1981, 84, 293; (b) A. M. Renlund, F. Shokoohi, H. Reisler and C. Wittig, J. Phys. Chem., 1982, 86, 4165.
27. H. Van Look and J. Peeters, J. Phys. Chem., 1995, 99, 16284.
28. K. Devriendt, H. Van Look, B. Ceursters and J. Peeters, Chem. Phys. Lett., 1996, 261, 450.
29. (a) J. Peeters, H. Van Look and B. Ceursters, J. Phys. Chem., 1996, 100, 15124; (b) K. Devriendt and J. Peeters, J. Phys. Chem. A, 1997, 101, 2546.
30. H. Thiesemann and C. A. Taatjes, Chem. Phys. Lett., 1997, 270, 580.
31. (a) A. M. Wodtke and Y. T. Lee, J. Phys. Chem., 1985, 89, 4744; (b) B. A. Balko, J. Zhang and Y. T. Lee, J. Chem. Phys., 1991, 94, 7958.
32. S. H. S. Wilson, C. L. Reed, D. H. Mordaunt, M. N. R. Ashfold and M. Kawasaki, Bull. Chem. Soc. Jpn., 1996, 69, 71.
33. T. R. Fletcher and S. R. Leone, J. Chem. Phys., 1989, 90, 871.
34. Y.-C. Hsu, J. J.-M. Lin, D. Papousek and J.-J. Tsai, J. Chem. Phys., 1993, 98, 6691.
35. Y.-C. Hsu, P. R. Wang, M.-C. Yang, D. Papousek, Y.-T. Chen and W.-Y. Chiang, Chem. Phys. Lett., 1992, 190, 507.
36. Y.-C. Hsu, Y.-J. Shiu and C. M. Lin, J. Chem. Phys., 1995, 103, 5919.
37. Y.-C. Hsu, F.-T. Chen, L.-C. Chou and Y.-J. Shiu, J. Chem. Phys., 1996, 105, 9153.
38. F. Shokoohi, T. A. Watson, H. Reisler, F. Kong, A. M. Renlund and C. Wittig, J. Phys. Chem., 1986, 90, 5695.
39. J. W. Stephens, J. L. Hall, H. Solka, W.-B. Yan, R. F. Curl and G. P. Glass, J. Phys. Chem., 1987, 91, 5740.
40. D. R. Lander, K. G. Unfried, J. W. Stephens, G. P. Glass and R. F. Curl, J. Phys. Chem., 1989, 93, 4109.
41. D. R. Lander, K. G. Unfried, G. P. Glass and R. F. Curl, J. Phys. Chem., 1990, 94, 7759.
42. S. K. Farhat, C. L. Morter and G. P. Glass, J. Phys. Chem., 1993, 97, 12789.
43. (a) J. O. P. Pedersen, B. J. Opansky and S. R. Leone, J. Phys. Chem., 1993, 97, 6822; (b) B. J. Opansky and S. R. Leone, J. Phys. Chem., 1996, 100, 19904.
44. B. J. Opansky, P. W. Seakins, J. O. P. Pedersen and S. R. Leone, J. Phys. Chem., 1993, 97, 8583.
45. J. E. Butler, J. W. Fleming, L. P. Goss and M. C. Lin, Chem. Phys., 1981, 56, 355.
46. I. W. M. Smith, J. Chem. Soc., Faraday Trans., 1997, 93, 3741.
47. (a) A. H. Laufer and A. M. Bass, J. Phys. Chem., 1979, 83, 310; (b) A. H. Laufer and R. Lechleider, J. Phys. Chem., 1984, 88, 66.
48. K. S. Shin and J. V. Michael, J. Phys. Chem., 1991, 95, 5864.
49. M. Koshi, K. Fukada, K. Kamiya and H. Matsui, J. Phys. Chem., 1992, 96, 9839.
50. S. J. Klippenstein and Y.-W. Kim, J. Chem. Phys., 1993, 99, 5790.
51. I. W. M. Smith, in The Chemical Kinetics and Dynamics of Small Radicals, ed. K. Liu and A. Wagner, World Scientific, Singapore, 1995, p. 214.
52. D. C. Clary, Annu. Rev. Phys. Chem., 1990, 41, 61.
53. I. W. M. Smith, Int. J. Mass Spectrom. Ion Proc., 1995, 149/150, 231.
54. I. W. M. Smith, Berichte Bunsen-Ges Phys. Chem., 1997, 101, 516.
55. E. Martinez, B. Cabañas, A. Aranda, J. Albaladijo and R. P. Wayne, J. Chem. Soc., Faraday Trans., 1997, 93, 2043 and references therein.
56. S. G. Lias, J. E. Bartmess, J. F. Liebmann, J. L. Holmes, R. D. Levin and W. G. Mallard, J. Phys. Chem. Ref. Data, 1988, 17, Suppl. 1.
57. W. M. Irvine, P. F. Goldsmith and A. Hjalmarson, in Interstellar Processes, ed. D. J. Hollenbach and H. A. Thronson Jr., Reidel, Dordrecht, 1987, p. 561.
58. D. L. Yang, T. Yu, M. C. Lin and C. F. Melius, J. Chem. Phys., 1992, 97, 222.
59. D. L. Yang and M. C. Lin, in ref. 51, p. 164.
60. Handbook of Chemistry and Physics, ed. D. R. Lide, CRC Press, Boca Raton, FL, 71st edn., 1990.