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Electron Spin Resonance of the Benzene Positive‐Ion Radical
1.(a) A. D. Liehr, Z. Physik. Chem. (Frankfurt) 9, 338 (1956);
1.(b) J. P. Colpa, Proc. Intern. Meeting Mol. Spectry., 4th Bologna, 1959, 1, 210 (1962);
1.(c) L. C. Snyder, J. Chem. Phys. 33, 619 (1960);
1.(d) A. D. Liehr, Z. Naturforsch. 16a, 641 (1961);
1.(e) L. C. Snyder, J. Phys. Chem. 66, 2299 (1962);
1.(f) C. A. Coulson and A. Golebiewski, Mol. Phys. 5, 71 (1962).
2.(a) W. D. Hobey and A. D. McLachlan, J. Chem. Phys. 33, 1695 (1960);
2.(b) A. D. McLachlan, Mol. Phys. 4, 417 (1961);
2.(c) H. M. McConnell and A. D. McLachlan, J. Chem. Phys. 34, 1 (1961);
2.(d) A. D. McLachlan and L. C. Snyder, J. Chem. Phys. 36, 1159 (1962);
2.(e) A. D. Liehr, Ann. Rev. Phys. Chem. 13, 41 (1962);
2.(f) W. D. Hobey, J. Chem. Phys. 43, 2187 (1965).
3.R. W. Fessenden and S. Ogawa, J. Am. Chem. Soc. 86, 3591 (1964).
4.(a) T. R. Tuttle and S. I. Weissman, J. Am. Chem. Soc. 80, 5342 (1958);
4.(b) G. K. Fraenkel et al. (private communication).
5.A. Carrington and I. C. P. Smith, Mol. Phys. 7, 99 (1963–64).
6.G. Vincow, M. L. Morrell, M. V. Volland, H. J. Dauben, Jr., and F. R. Hunter, J. Am. Chem. Soc. 87, 3527 (1965).
7.(a) T. J. Katz and H. L. Strauss, J. Chem. Phys. 32, 1873 (1960);
7.(b) H. L. Strauss, T. J. Katz, and G. K. Fraenkel, J. Am. Chem. Soc. 85, 2360 (1963).
8.(a) J. dos Santos‐Veiga, Mol. Phys. 5, 639 (1962);
8.N. L. Bauld and M. S. Brown, J. Am. Chem. Soc. 87, 4390 (1965);
8.(b) S. H. Glarum and J. H. Marshall, J. Chem. Phys. 41, 2182 (1964);
8.(c) P. H. H. Fischer and C. A. McDowell, Mol. Phys. 8, 357 (1964);
8.(d) A. Carrington, H. C. Longuet‐Higgins, and P. F. Todd, Mol. Phys. 9, 211 (1965);
8.(e) M. T. Jones, A. Cairncross, and D. W. Wiley, J. Chem. Phys. 43, 3403 (1965).
9.A number of interesting experiments have also been performed on the highly symmetrical radicals in the solid state: (a) H. J. Silverstone, D. E. Wood, and H. M. McConnell, J. Chem. Phys. 41, 2311 (1964);
9.(b) G. R. Liebling and H. M. McConnell, J. Chem. Phys. 42, 3931 (1965);
9.(c) M. T. Jones, J. Am. Chem. Soc. 88, 174 (1966).
10.(a) M. G. Townsend and S. I. Weissman, J. Chem. Phys. 32, 309 (1960);
10.(b) J. H. Freed, J. Chem. Phys. 43, 1427 (1965).
11.(a) H. M. McConnell, J. Chem. Phys. 34, 13 (1961);
11.(b) J. R. Bolton and A. Carrington, Mol. Phys. 4, 271 (1961);
11.(c) M. T. Jones, J. Chem. Phys. 42, 4054 (1965).
12.B. G. Segal, M. Kaplan, and G. K. Fraenkel, J. Chem. Phys. 43, 4191 (1965).
13.(a) T. R. Tuttle, Jr. and S. I. Weissman, J. Am. Chem. Soc. 80, 5342 (1958);
13.(b) V. V. Voevodskii, S. P. Solodovnikov, and Chibrikin, Dokl. Akad. Nauk 129, 1082 (1959);
13.(c) J. R. Bolton and A. Carrington, Mol. Phys. 4, 497 (1961);
13.(d) T. R. Tuttle, Jr., J. Am. Chem. Soc. 84, 1492, 2839 (1962);
13.(e) J. R. Bolton, A. Carrington, A. Forman, and L. E. Orgel, Mol. Phys. 5, 43 (1962);
13.(f) J. R. Bolton, J. Chem. Phys. 41, 2455 (1964).
14.G. Vincow et al. (unpublished work).
15.(a) A. Carrington and P. F. Todd, Mol. Phys. 7, 533 (1963–1964);
15.(b) A. Carrington and P. F. Todd, Mol. Phys. 8, 299 (1964).
16.Very interesting results on the effect of deuterium substitution in the nearly degenerate radicals have been reported: (a) R. G. Lawler, J. R. Bolton, G. K. Fraenkel, and T. H. Brown, J. Am. Chem. Soc. 86, 520 (1964);
16.(b) A. Carrington, H. C. Longuet‐Higgins, R. E. Moss, and P. F. Todd, Mol. Phys. 9, 187 (1965);
16.(c) M. Karplus, R. G. Lawler, and G. K. Fraenkel, J. Am. Chem. Soc. 87, 5260 (1965).
17.(a) T. H. Brown, M. Karplus, and J. C. Schug, J. Chem. Phys. 38, 1749 (1963);
17.(b) T. H. Brown and M. Karplus, J. Chem. Phys. 39, 1115 (1963).
18.W. D. Hobey, Mol. Phys. 7, 325 (1963–1964).
19.R. L. Flurry, Jr. and P. G. Lykos, Mol. Phys. 6, 283 (1963).
20.H. M. McConnell, J. Chem. Phys. 28, 1188 (1958).
21.(a) I. Bernal, P. H. Rieger, and G. K. Fraenkel, J. Chem. Phys. 37, 1489 (1962);
21.(b) G. Berthier, A. Viellard, and G. Del Re, Phys. Letters 8, 313 (1964);
21.(c) T. H. Brown, J. Chem. Phys. 41, 2223 (1964).
22.M. R. Das and G. K. Fraenkel, J. Chem. Phys. 42, 792 (1965).
23.I. A. Zlochower, W. R. Miller, Jr., and G. K. Fraenkel, J. Chem. Phys. 42, 3339 (1965).
24.J. R. Bolton, Mol. Phys. 6, 219 (1963).
25.H. L. Strauss and G. K. Fraenkel, J. Chem. Phys. 35, 1738 (1961).
26.M. Karplus and G. K. Fraenkel, J. Chem. Phys. 35, 1312 (1961).
27.J. R. Bolton and G. K. Fraenkel, J. Chem. Phys. 40, 3307 (1964).
28.A. J. Stone, Mol. Phys. 6, 509 (1963);
28.A. J. Stone, 7, 311 (1964)., Mol. Phys.
29.F. Hughes, R. D. Kirk, and F. W. Patten, J. Chem. Phys. 40, 872 (1964).
30.Varian Associates, “EPR at Work Series,” No. 28.
31.J. S. Hyde and H. W. Brown, J. Chem. Phys. 37, 368 (1962);
31.J. S. Hyde (private communication).
32.H. Margenau and G. M. Murphy, The Mathematics of Physics and Chemistry (D. Van Nostrand Co., Inc., Princeton, N.J., 1956), 2nd ed., p. 519.
33.The hyperfine spacing and g value are measured relative to those of Fremy’s salt (see Sec. II).
34.We are grateful to the referee for suggesting the interesting possibility that this underlying resonance arises from the benzene dimer radical cation, 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)].
34.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)] is 2.33 G in good agreement with experiment.
35.G. Vincow and P. M. Johnson, J. Chem. Phys. 39, 1143 (1963).
36.The hyperfine spacing and g value have been measured relative to those of the napthacene monopositive ion (see Sec. II).
37.M. Kasha, J. Opt. Soc. Am. 38, 1068 (1948).
38.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 γ 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 in a sample of photoionized benzene in sulfuric acid.
39.Lawler et al. (Ref. 16a) investigated in solution. The small nonequivalence of splittings observed in that work is not expected to be detectable in our spectrum which exhibits broad overlapped components.
40.J. R. Bolton and A. Carrington, Proc. Chem. Soc. (London) 1961, 174.
41.Reference 40, p. 385.
42.S. Ohnishi and I. Nitta, J. Chem. Phys. 39, 2848 (1963).
43.(a) P. Bennema, G. J. Hoijtink, J. H. Lupinski, L. J. Ooster‐hoff, P. Seiler, and J. D. W. van Voorst, Mol. Phys. 2, 431 (1959);
43.J. H. Lupinski, thesis, University of Leiden (1959);
43.(b) W. C. Meyer and A. C. Albrecht, J. Phys. Chem. 66, 1168 (1962);
43.K. D. Cadogan and A. C. Albrecht, J. Chem. Phys. 43, 2550 (1965).
44.The spectra which decay at are those of the toluene and β‐methylnaphthalene cations and the corresponding underlying resonances are due to the neutral benzyl and ß‐methylnaphthenyl radicals. Evidence that the benzyl radical gives rise to the underlying resonance in the case of uv‐illuminated toluene has been obtained from consideration of the magnitude of the second moment of the spectrum and of its total extent (Ref. 35). In addition, the ESR of benzyl radical in solution has recently been investigated [A. Carrington and I. C. P. Smith, Mol. Phys. 9, 137 (1965);
44.R. O. C. Norman (private communication)], and the second moment predicted from the isotropic proton hyperfine splittings found to be in very good agreement with that measured in the polycrystalline medium.
45.The data of Silverstone, Wood, and McConnell on in thiourea (Table I of Ref. 9a) indicate clearly that there is a temperature dependence of the proton hyperfine spacing. Related measurements are being made in our laboratory in order to establish this effect and compare its magnitude with our temperature dependence results for in solution (Ref. 6).
46.Further confirmation for this approximation can be inferred from the results of an investigation of in a polycrystalline medium (Ref. 9c).
47.(a) A. Carrington, F. Dravnieks, and M. C. R. Symons, J. Chem. Soc. 1959, 947;
47.(b) I. C. Lewis and L. S. Singer, J. Chem. Phys. 43, 2712 (1965).
48.(a) J. R. Bolton and G. K. Fraenkel, J. Chem. Phys. 40, 3307 (1964);
48.(b) A. D. McLachlan, Mol. Phys. 2, 271 (1959).
49.(a) J. R. Bolton, J. Chem. Phys. 43, 309 (1965);
49.(b) J. P. Colpa and J. R. Bolton, Mol. Phys. 6, 273 (1963);
49.(c) J. Higuchi, J. Chem. Phys. 39, 3455 (1963);
49.(d) G. Giacometti, P. L. Nordio, and M. V. Pavan, Theoret. Chim. Acta 1, 404 (1963).
50.The application of this treatment to the various radicals has been considered and will be reported elsewhere [G. Vincow, “ESR of Cyclic Radicals Relationship of Total Splitting to Parameters of the Calpa‐Bolton and Giacometti‐Nordio‐Pavan Equations,” J. Chem. Phys. (to be published)].
51.L. C. Snyder and T. Amos, J. Chem. Phys. 42, 3670 (1965).
52.A. D. McLachlan, H. H. Dearman, and R. Lefebvre, J. Chem. Phys. 33, 65 (1960).
53.The experiments which have been performed on lend further weight to this argument since the temperature coefficient for C‐13 hyperfine splitting in this radical has been found to be positive (Ref. 6). It is also of interest that the linewidths in are independent of temperature. The linewidths in increase with increasing temperature indicating the possibility of a contribution from exchange effects [J. R. Bolton (private communication)].
54.D. M. Schrader and M. Karplus, J. Chem. Phys. 40, 1593 (1964).
55.The hyperfine constant averaged over the zero‐point motion is less than 2% larger than the value for a planar rigid radical, a gratifying result which indicates that large vibrational corrections may not be needed in the interpretation of the proton hyperfine spectra of planar hydrocarbon radicals at room temperature.
56.M. Karplus (private communication).
57.H. C. Longuet‐Higgins, U. Öpik, M. H. L. Pryce, and A. Sack, Proc. Roy. Soc. (London) A244, 1 (1958).
58.Details of the calculation for would be different, since the unpaired‐electron molecular orbital is not the same as in
59.G. Herzberg, Molecular Spectra and Molecular Structure, II. Infrared and Raman Spectra of Polyatomic Molecules (D. Van Nostrand Co., Inc., N.J., 1945).
60.This perturbation is also responsible for “spin‐switching” effects which are held to result in the uniform time‐average proton splitting (Ref. 2c).
61.The contribution of the doubly excited vibration is small since it is weighted by a Boltzmann factor of
62.If the C‐H out‐of‐plane bending motions dominate the temperature effect then the increased density of states may actually favor a smaller temperature effect since there are numerous vibrational levels within of the ground state with the C‐H perpendicular bending modes in their zero‐point states. Further, the excited state of the C‐H bending motion at is estimated to possess a smaller amplitude of vibration than the corresponding state of in the case of methyl radical.
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