COMMUNICATIONS TO THE EDITOR
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greater mobility than any doubly charged ion in glass, and its presence on the salt side of the double layer could introduce complicating structural modifications. SCHOOL O F CHEMISTRY RUTGERS UNIVERSITY NEW BRUNSWICK, NEWJERSEY 08903
RICHARD W. LAITY CARLAXELSJOBLOM
RECEIVED JULY 13, 1967
New Electron Spin Resonance Spectra in 5000 G
y-Irradiated Alkyl Halides at 77"KI 4900 G
Sir: The reported esr spectra of most alkyl halide glasses which have been y irradiated at 77°K are attributable to the alkyl radical formed byrupture of the C-X b ~ n d , ~ indicating -~ that other paramagnetic species (such as X, Xz-, RXaX, RX+, RX-) postulated to account for portions of the optical spectra5ss are present at very low concentrations or are obscured by line-broadening effects. Relatively complex spectra observed from some polycrystalline alkyl iodides4*'and some alkyl bromide^^!^ have been attributed to spinorbit coupling involving the halogen nucleus in halogencontaining radicals. Using esr sensitivities ranging up to several hundred times that required to give full-scale deflection for alkyl radicals produced by radiolysis of glassy alkyl halides, we have found that all ?-irradiated glassy and polycrystalline alkyl iodides and bromides tested give esr spectra at much lower and higher magnetic fields than previously observed. The low-field spectra fall into four types corresponding to spectra A, B, C, and D of Figure 1: (A) polycrystalline CzH51,(B) polycrystalline n-C4HJ, (C) polycrystalline n-CXH71, n-C5H91, n-CeHJ, n-C7H15I, and (D) all glassy and polycrystalline n-alkyl bromides from Cz through C,. The low-field spectra of alkyl iodide glasses are similar to the type C spectra after some change in the relative peak heights of the latter during standing at 77°K. All of the alkyl iodides, in either the glassy Or polycrystallil'e give highspectra to E. The high-fie1d spectra Of the bromides are all similar to F. ?-Irradiated polycrystalline n-alkyl chlorides from Cz through C6 give no esr spectra at fields more than 250 gauss below or above the free electron g value, although some Structure above and below the alkyl radical signal is present in every case. Glassy chlorides, when obtainable, give similar results. For tested the features (in the region of 3260 gauss) decay at 77°K. The Journal of' Physical Chemistry
Figure 1. Low-field (A, B, C, D) and high-field (E, F) esr spectra of 7-irradiated polycrystalline alkyl iodides and bromides a t 77°K: (A) CtHJ, signal level 400; (B) n-C4&,1, signal level 400; (C) n-C5H111, signal level 400; (D) n-C4HgBr, the three sections of the spectrum being measured a t different signal levels as indicated; (E) C2HJ, signal level 2500; (F) n-C5HllBr, signal level 1000. I n all cases the modulation amplitude was about 12 gauss and the y dose was 4 x 1019 ev g-1. The magnetic field corresponding t o the free-electron g value is ca. 3260 gauss.
However, some portions of the low- and high-field spectra grow while others decay, the growth in CzH51 glass being as much as fourfold in 2 days. The low- and high-field spectra must be caused by halogen-containing species. Consistent with this conclusion are the differences between the spectra of iodides, bromides, and chlorides. The low- and highfield spectra of polycrystalline and glassy C2D51are identical with those of Cz&,I,indicating that the observed line widths and line splittings are not controlled by hydrogen. (1) This work was supported in part by the United States Atomic Energy Commission under Contract AT(ll-1)-1715 and by the W. F. Vilas Trust of the University of Wisconsin. (2) B. Smaller and M. S. Matheson, J. Chem. Phys.. 28, 1169 (1958). (3) P. B. Ayscough and C. Thompson, Trans. Faraday Soc., 1477 (1962). (4) H. W. Fenrick, S. V. Filseth, A. L. Hanson, and J. E. Willard, J . Am. Chem. Soc., 85, 3731 (1963). (5) (a) E. P. Bertin and W. H. Hamill, ibid., 86, 1301 (1964); (b) J. P. Mittal and W. H. Hamill, ibid., 89,5749 (1967). (6) R. F. c. Claridge and J. E. Willard. ibid., 88, 2404 (1966). (7) H. W. Fenrick and J. E. Willard, ibid., 88, 412 (1966). (8) F. W. Mitchell, B. C. Green, and J. W. T. Spinks, J. Chem. Phy8., 36, 1095 (1962). (9) R. M. A. Hahne and J. E. Willard, J. Phys. Chem., 68, 2582 (1964).
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COMMUNICATIONS TO THE EDITOR
Among the species which may contribute to the spectra of Figure 1 are X, Xz-, X2+,RXaX, R X f , and RX-. The esr spectrum of gaseous I atoms1° extends from 3300 to 6223 gauss. We find a spectrum extending from 1500 to 5000 gauss from yirradiated polycrystals prepared by fusing KI with 0.1 mole % AgN03, a medium in which the optical spectrum has been attributed to IZ-.l1 The esr spectrum of Br2- in Xirradiated KBr single crystals containing alkaline earth ions is observed12between 2000 and 4800 gauss. Samples were irradiated to a dose of 4 X l O l 9 ev g-’ a t a dose rate of 2 X 10ls ev g-’ min-l. Magnetic field values were assigned with a Varian Fieldial accessory. The modulation amplitude of 4000 (about 12 gauss) used gave a maximum esr signal without distortion. The other experimental methods used have been described.“
modes of forming or breaking bonds accessible, in principle, to such compounds as N20, 0 2 , and GO2. Nevertheless, certain trial postulates (outlined below) have been formulated which provide remarkably good agreement between computed and measured Eo, and, accordingly, show promise of adding to our understanding of the nature of bimolecular reactions. These Eo computations do not use adjustable parameters, but rely on bond properties such as dissociation energy and vibrational frequency. For the multivalent bimolecular B =A BX, one of the trial transfer reaction AX postulates relating the bond order nz (in the transition state A. * .X. .B) of X . . B to the bond order nl of A . - + Xcan be written
+
n2
+
= (n”Bx/n”Ax)(n”Ax
- nl)
(1)
where n”Ax is the bond order of the reactant AX, and n ” B X is the bond order of the product BX. Equation 1 (10) S. Aditya and J. E. Willard, J . Chem. Phya., 44, 833 (1966). meets the requirement that nl is n”Ax when n2is zero (11) C. J. Delbecq, W.Hayes, and P. H. Yuster, Phys. Rev.,1 2 1 , at the initiation of the reaction, and vice versa. For 1043 (1961). univalent reactions, eq 1 reduces to the previously (12) W. Hayes and G. M. Nichols, ibid., 117, 993 (1960). used1~2~4 trial postulate that nl n2 = 1. CorrespondDEPARTMENT OF CHEMISTRY RICHARD J. EGLAND 1 is equivalent to the simple postulate that ingly, eq UNIVERSITY OF WISCONSIN JOHN E. WILLARD the bond-order increase of the forming bond is linearly MADISON, WISCONSIN 53706 related to the decrease in bond order of the breaking RECEIVED JULY17, 1967 bond, with the proportionality constant equal to the ratio of the bond order of BX to that of AX. As in univalent calculations, the potential energy of activaV , in the transition state is taken as the energy tion, Computed Activation Energies for Bimolecular lost by the bond AX as it dissociates to A . . . X of bond Reactions of 0 2 , Nz, NO, NzO, NOZ,and C02 order nl, less the energy supplied by the formation of B . . . X of bond order n2, plus a repulsive energy V , Sir: Although it had once been hoped that absolute arising from parallel electron spins on -4and B. rate theory would permit useful kinetic predictions for = D e , ~x D’Ax(ni)lrAXmost reactions, the problem of predicting activation energies is still one of the major unsolved questions in D ’ x B ( ~ ~ ) *V~, kcal/mole ~ (2) chemistry since tractable accurate quantum-mechanical where D e , ~ Xis the dissociation potential energy of AX, solutions do not exist for potential energy surfaces of D’ is the dissociation potential energy of the single reactions involving multielectron atoms. For hydrobond, and V , is obtained from eq 10 of ref 4. The gen atom reactions in the gas phase, it has been shown, A is log (D,/D’)/log n”, which is the slope exponent however, that transition-state computations’, of activaof the log dissociation energy us. log bond order line. tion energies and rate constants by a nonquantum n1 or n2 in eq 2 is less than one, A is replaced by When method can exhibit good agreement with experiment the slope p , calculated’ for the bond-order region below when several tentative assumptions are used to provide one. As in previous investigation^,'^^^^ the computer interrelationships (such as Pauling’s rule3) among bond program (modified for multivalent bonds) determines energy, bond order, and bond length. In an effort to avoid this computing method’s limitation to H-atom (1) H. S. Johnston and C. Parr, J . Am. Chem. Soc., 85, 2544 (1963). reactions, a reduced-variable treatment was devised4 (2) S. W. Mayer, L. Schieler, and H. S. Johnston, J . Chem. Phys., for other univalent atoms, such as the halogens. 45, 385 (1966). The calculation of activation energies for the im(3) L. Pauling, “The Nature of the Chemical Bond,” 3rd ed, Cornel1 University Press, Ithaca, N. Y . , 1960. portant class of bimolecular reactions involving multi(4) S. W. Mayer, L. Schieler, and H. 5. Johnston, “Proceedings of valent bonds has been an even more difficult challenge the Eleventh International Symposium on Combustion,” The Combecause of the complexities introduced by the many bustion Institute, Pittsburgh, Pa., 1967, p 837.
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Volume 71, Number 1.9 November 1967
