Combined 2H and 18O isotope effects in support of a concerted, synchronous elimination of acetaldehyde from a bis(benzyl ethyl ether) radical cation

Journal of the American Chemical Society

Volumn 110, Issue 19, 1988, Pages 6291-6297

  Stringer, Michael B ^a   Bowie, John H ^a   Allison, Colin E ^b   Derrick, Peter J ^c  

^a UNIVERSITY OF ADELAIDE   (Australia)

^b UNIVERSITY OF NEW SOUTH WALES   (Australia)

^c UNIVERSITY OF WARWICK   (United Kingdom)

Author keywords

[No Author keywords available]

Indexed keywords

EID: 0141670698     PISSN: 00027863     EISSN: 15205126     Source Type: Journal    
DOI: 10.1021/ja00227a002     Document Type: Article

References (20)

1. Stone, D. J. M.; Bowie, J. H.; Underwood, D. J.; Donchi, K. F.; Allison, C. E.; Derrick, P. J. J. Am. Chem. Soc. 1983, 105, 1688.
2. Wesdemiotis, C.; Feng, R.; McLafferty, F. W. J. Am. Chem. Soc. 1985, 107, 715.
3. McLafferty, F. W. Anal. Chem. 1956, 28, 306. (b) McLafferty, F. W. Anal. Chem. 1959, 31, 82. (c) Kingston, D. G. I.; Bursey, J. T.; Bursey, M. M. Chem. Rev. 1974, 2, 215.
4. Gilpin, J. A.; McLafferty, F. W. Anal. Chem. 1957, 29, 990. (b) Liedtke, R. J.; Djerassi, C. J. Am. Chem. Soc. 1969, 91, 6814. (c) Fenselau, C.; Young, J. L.; Meyerson, S.; Landis, W.; Selke, E.; Leitch, L. C. J. Am. Chem. Soc. 1969, 91, 6847. (d) Derrick, P. J.; Falick, A. M.; Lewis, S.; Burlingame, A. L. J. Phys. Chem. 1979, 83, 1567.
5. Boer, F. P.; Shannon, T. W.; McLafferty, F. W. J. Am. Chem. Soc. 1968, 90, 7239.
6. Witiak, D. N.; McLafferty, F. W.; Dill, J. D. J. Am. Chem. Soc. 1978, 100, 6639. (b) McAdoo, D. J.; Hudson, C. E. J. Am. Chem. Soc. 1981, 103, 7710. (c) Weber, R.; Levsen, K.; Wesdemiotis, C.; Weiske, T.; Schwarz, H. Int. J. Mass Spectrom. Ion Phys. 1982, 43, 131. (d) Zwinselman, J. J.; Nibbering, N. M. M.; Hudson, C. E.; McAdoo, J. J. Int. J. Mass Spectrom. Ion Phys. 1983, 47, 129.
7. Hammerum, S. Mass Spectrom. Rev. 1988, 7, 123. (b) McAdoo, D. J.; Hudson, C. E. J. Am. Chem. Soc. 1981, 103, 7710.
8. Dewar, M. J. S. J. Am. Chem. Soc. 1984, 106, 209.
9. Darcy, M. G.; Rogers, D. E.; Derrick, P. J. Int. J. Mass Spectrom. Ion Phys. 1978, 27, 335. (b) Cullis, P. G.; Neumann, G. M.; Rogers, D. E.; Derrick, P. J. Adv. Mass Spectrom. 1980, 8, 1729.
10. Wenner, W. J. Org. Chem. 1952, 17, 523. This method gives a mixture of dibromo and tribromo compounds. We could only obtain pure 1,4-bis(bromomethyl)benzene by repeated crystallization from ethanol (yield 18%).
11. Murray, A.; Williams, D. L. Organic Syntheses with isotopes; Interscience: New York, 1958; Part II, p 1330.
12. Rumpf, B. A.; Allison, C. E.; Derrick, P. J. Org. Spectrom. 1986, 21, 295.
13. Measurements were made at La Trobe University by Dr. J. C. Traeger. For a description of the technique, see: Traeger, J. C.; McLoughlin, R. G. Int. J. Mass Spectrom. Ion Phys. 1978, 27, 319.
14. 0) is the number of states in the transition state, and N(E) is the density of states in the reactant. See: Forst, W. Theory of Unimolecular Reactions; Academic: New York, 1973. The algorithm employed to count states and the computer programs have been described: Allison, C. E. Ph.D. Thesis, University of New South Wales, 1986. (Copies of this thesis are available upon request from the Librarian, University of New South Wales, P.O. Box 1, Kensington, NSW 2033, Australia.) This expression for the microcanonical rate constant k(E) was derived independently at about the same time by Rosenstock et al. (Rosenstock, H. M.; Wallenstein, M. B.; Wahrhaftig, A. L.; Eyring, H. Proc. Natl. Acad. Sci. U.S.A. 1952, 38, 667) and by Marcus (Marcus, R. A. J. Chem. Phys. 1952, 20, 359). Applied in a straightforward fashion, the absolute values of rate constants calculated using the expression have been shown to be in satisfactory agreement with experiment for a variety of large organic ions and K(E)'s in the range 10 4-10 7 s -1. See, for example: Baer, T.; Willett, G. D.; Smith, D.; Phillips, J. S. J. Chem. Phys. 1979, 70, 4076.) Our application is less demanding of the theory, because the concern is with relative rates and with parallel reactions of a single reactant occurring over identical potential energy surfaces. This places constraints upon adjustments of vibrational frequencies and critical energies. For a discussion, see: Derrick, P. J.; Donchi, K. F. In Comprehensive Chemical Kinetics; Bamford, C. H., Tipper, C. F. H., Eds.; Elsevier: Amsterdam, the Netherlands, 1983; Vol. 24, pp 53–247.
15. Bingham, R. C.; Dewar, M. J. S.; Lo, D. H. J. Am. Chem. Soc. 1975, 97, 1285. (b) Dewar, M. J. S.; Thiel, W. J. J. Am. Chem. Soc. 1977, 99, 4899.
16. Derrick, P. J.; Donchi, K. F. In Comprehensive Chemical Kinetics; Bamford, C. H., Tipper, C. F. H., Eds.; Elsevier: Amsterdam, the Netherlands, 1983; Vol. 24.
17. Allison, C. E. Ph.D. Thesis, University of New South Wales, Australia, 1986.
18. See: Derrick, P. J.; Falick, A. M.; Burlingame, A. L.; Djerassi, C. J. Am. Chem. Soc. 1974, 96, 1054, 1059. The stabilization of the distonic ion intermediate in the Mclafferty rearrangement in a ketone shifts “hydrogen scrambling” to shorter times (nanoseconds) rather than to longer times.
19. These theoretical analyses point to the need for care in applying the “double-fractionation argument” (Belasco, J. G.; Albery, W. J.; Knowles, J.R. J. Am. Chem. Soc. 1983, 105, 2475) to unimolecular reactions of these ether molecular ions. According to this argument, the 16O/18O isotope effect should theoretically fall on replacing H by D in the case of a stepwise reaction but remain the same in the case of a synchronous reaction. In contrast to the enzyme reactions for which double-fractionation has been demonstrated unimolecular reactions of isolated ions may not occur “at a temperature” as such. With a stepwise enzyme reaction, the intermediate can be assumed to be at the temperature of the system, provided the intermediate's lifetime is long enough. With an isolated reaction, the internal energy of an intermediate in a stepwise process is dependent upon, but generally not the same as, the internal energy of the reactant. In the particular case of the ether molecular ions, the small fall in the 36O/18O isotope effect predicted for both the stepwise and the concerted model on replacing H by D is in both cases largely ar internal energy effect.
20. Robinson, P. J.; Holbrook, K. A. Unimolecular Reaction; Wiley-Interscience: London, 1972.