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Journal of Chemical Physics / Volume 47 / Issue 1

J. Chem. Phys. 47, 292 (1967); http://dx.doi.org/10.1063/1.1711862 (11 pages)

Electron Spin Resonance of the Benzene Positive‐Ion Radical

Melvin Keith Carter and Gershon Vincow

Department of Chemistry, University of Washington, Seattle, Washington

(Received 6 June 1966)

The benzene monopositive‐ion radical C6H6+ is prepared by photoionization of benzene in a rigid sulfuric acid matrix. This highly symmetrical radical is of special interest because of the occurrence of near degeneracy in the vibronic levels. Electron spin resonance spectra are investigated over a range of temperatures. The hyperfine spacing is 4.44±0.01 G at −150°C and varies with temperature. The temperature coefficient of the total extent of the spectrum is −5.2±3 mG/°C (−110≤t≤−190°C). A computation of the benzene cation splitting using the semiempirical Colpa—Bolton equation [J. Chem. Phys. 43, 309 (1965)], which treats the effect of excess charge density on hyperfine splitting, leads to a value in fairly good agreement with experiment. The temperature dependence of the proton hyperfine splitting is discussed in terms of the expectation values of the Fermi contact interaction in the thermally populated molecular vibronic states. The g value observed for C6H6+ is 2.00242±0.00002. This result is compared with the g value computed from Stone's semiempirical equation [Mol. Phys. 6, 509 (1963); J. Chem. Phys. 43, 4191 (1965)]. Near degeneracy in hydrocarbon radicals is correlated with the occurrence of deviations from the predictions of Stone's equation.

© 1967 American Institute of Physics

ARTICLE DATA

Digital Object Identifier
http://dx.doi.org/10.1063/1.1711862

PUBLICATION DATA

ISSN

0021-9606 (print)  
1089-7690 (online)

Publisher

$abstract.society.value is a Member of CrossRef American Institute of Physics

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    References

    (a) M. G. Townsend and S. I. Weissman, J. Chem. Phys. 32, 309 (1960)JCPSA6000032000001000309000001;, (b) J. H. Freed, J. Chem. Phys. 43, 1427 (1965)JCPSA6000043000004001427000001.

    B. G. Segal, M. Kaplan, and G. K. Fraenkel, J. Chem. Phys. 43, 4191 (1965)JCPSA6000043000012004191000001.

    (a) T. H. Brown, M. Karplus, and J. C. Schug, J. Chem. Phys. 38, 1749 (1963)JCPSA6000038000007001749000001;, (b) T. H. Brown and M. Karplus, J. Chem. Phys. 39, 1115 (1963)JCPSA6000039000004001115000001.

    H. M. McConnell, J. Chem. Phys. 28, 1188 (1958)JCPSA6000028000006001188000001.

    M. R. Das and G. K. Fraenkel, J. Chem. Phys. 42, 792 (1965)JCPSA6000042000002000792000001.

    I. A. Zlochower, W. R. Miller, Jr., and G. K. Fraenkel, J. Chem. Phys. 42, 3339 (1965)JCPSA6000042000009003339000001.

    H. L. Strauss and G. K. Fraenkel, J. Chem. Phys. 35, 1738 (1961)JCPSA6000035000005001738000001.

    M. Karplus and G. K. Fraenkel, J. Chem. Phys. 35, 1312 (1961)JCPSA6000035000004001312000001.

    J. R. Bolton and G. K. Fraenkel, J. Chem. Phys. 40, 3307 (1964)JCPSA6000040000011003307000001.

    F. Hughes, R. D. Kirk, and F. W. Patten, J. Chem. Phys. 40, 872 (1964)JCPSA6000040000003000872000001.

    We are grateful to the referee for suggesting the interesting possibility that this underlying resonance arises from the benzene dimer radical cation, (C6H6)2+. Such a dimer radical has recently been observed in the case of naphthalene [I. C. Lewis and L. S. Singer, J. Chem. Phys. 43, 2712 (1965)JCPSA6000043000008002712000001]., The peak heights of the underlying resonance are not in good agreement with intensities predicted for the cation dimer but the splitting constant computed using the Colpa-Bolton equation [J. Chem. Phys. 43, 309 (1965)JCPSA6000043000001000309000001] is 2.33 G in good agreement with experiment.

    G. Vincow and P. M. Johnson, J. Chem. Phys. 39, 1143 (1963)JCPSA6000039000005001143000001.

    The exchange rate may be enhanced by the photolysis of the sulfuric acid solvent to form hydrogen atoms. Trapped hydrogen atoms previously have been studied in the gamma radiolysis of sulfuric acid [see H. Zeldes and R. Livingston, Phys. Rev. 96, 1702 (1954)]. We have indeed observed the ESR of hydrogen atoms at −184 °C in a sample of photoionized benzene in sulfuric acid.

    S. Ohnishi and I. Nitta, J. Chem. Phys. 39, 2848 (1963)JCPSA6000039000011002848000001.

    L. C. Snyder and T. Amos, J. Chem. Phys. 42, 3670 (1965)JCPSA6000042000010003670000001.

    A. D. McLachlan, H. H. Dearman, and R. Lefebvre, J. Chem. Phys. 33, 65 (1960)JCPSA6000033000001000065000001.

    D. M. Schrader and M. Karplus, J. Chem. Phys. 40, 1593 (1964)JCPSA6000040000006001593000001.


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