1368
P. Neta and Robert H. Schuler
Substituent Effects on Electron Spin Resonance Parameters of Benzyl Radicals' P. Neta and Robert H. Schuler" Radiation Research Laboratories, Cenfer for Speciai Studies and Department of Chemistry, Melion lnstitute of Science, Carnegie-Mellon University, Pittsburgh, Pennsylvania 15213 (Received January 9, 1973) Pubiication costs assisted by Carnegie-Meiion University and the U . S. Atomic Energy Commission
The esr spectra of benzyl radicals substituted on the ring by COz-, 0-, SOs-, CH30, CH3, CHsCO, CN, F, or C1 have been observed during the radiolysis of aqueous solutions of the appropriate derivatives of toluene a t pH 13.7. All of these radicals show esr parameters very similar to those of benzyl radical. The extremes of the values for the CH2 hyperfine constants are 14.91 and 16.43 G for the-para 0- and F derivatives (16.34 G for benzyl) with most values, however, being in excess of 15.64 G. Effects of substitution on the hyperfine constants of the ring protons are extremely small. The smallness of the effects of substitution on the hyperfine constants shows that these various groups cause only a very limited perturbation of the electron spin distribution in the aromatic system. While the accuracy of the measurements is considerably greater than the small differences noted, it was not possible to derive any significant correlation with available substituent constants nor with theoretical calculations of the effects of substitution on spin distribution.
Introduction Substituent effects on the electron spin resonance parameters of aromatic radicals have been recently reviewed by Janzen.2 Correlations have been found between the substituent Hammett u values and the hyperfine constants of a series of substituted aromatic semidiones, nitro anions, nitroxides, and certain other nitrogen-centered radicals. The spin probes in all of these radicals have electronic properties which strongly influence the electron spin distribution. Since it seemed that substituted benzyl radicals would simulate more closely the molecules themselves, we have measured the hyperfine constants for various substituted benzyl radicals for the purpose of attempting a correlation with the substituent u values. These benzyl radicals have considerably shorter lifetimes than the radicals summarized by Janzen and can be studied only by continuously producing them during the course of the esr observations. We have chosen the in situ radiolysis method developed by Eiben and Fessenden3 for this purpose. Benzyl radicals can be produced in irradiated aqueous solutions by the reaction of 0- with substituted toluenes at high pH, as has been recently demonstrated.4 Although OH radicals add to an aromatic ring much more rapidly than they abstract hydrogen from a n aliphatic side chain, the situation is reversed when OH is converted into 0(pK = 11.9).5 At pH >13 the main reaction in irradiated solutions of substituted toluenes involves abstraction of H from the methyl group to produce the corresponding benzyl radicals.
Experimental Section Most of the organic compounds used were of the purest grade available from Aldrich and from Eastman. Toluene was a Matheson Coleman and Bell Spectrograde reagent and p-xylene was Research Grade from Phillips. Saturated or 0.01 M solutions were prepared in doubly distilled water containing 0.5 M Baker Analyzed KOH. The solutions were also saturated with NzO in order to convert The Journal of Physical Chemistry, Vol. 77,No. 1 I, 1973
eaq- to 0-. They were irradiated by 2.8-MeV electrons while flowing through a silica cell located within the esr cavity. The details of these experiments are similar to those previously described.3
Results and Discussion The spectra of the radicals reported in Table I were uncomplicated by the presence of other radicals.6 The spectral patterns consist of from 27 lines (for benzyl substituted with COz-, 0-, and SO3- at the para position) to 108 lines (for the para derivatives containing CH3 groups). The signal-to-noise ratio for the least intense spectra was -2: 1, with greater intensities being observed for radicals with less complex patterns and for those bearing negative charges which decrease the radical decay rates. The line widths were -0.1 G. The parameters calculated from the spectra are summarized in Table I. The present results for benzyl are in very good agreement with the hyperfine constants reported by Dixon and Norman7 and by Fischer.8 Except for p-carboxybenzyl none of the other radicals have been previously observed by esr . It is seen in Table I that substitution of an aromatic position produces no large effect on either the CH2 proton hyperfine constant or that of any of the ring protons. The largest changes are observed for the radicals p CHzCsH40- and p-CH2C6H&OCH3. In these cases the CHz coupling cohstants are only -10% less than that for benzyl. In all other cases the CH2 coupling constants range between 15.64 (p-CH2CsH4CN) and 16.43 G (pSupported in part by the U S . Atomic Energy Commission. E. G. Janzen, Accounts Chem. Res., 2,279 (1969). K. Eiben and R W.Fessenden.J. Phys. Chem., 75,1186 (1971). P. Neta, M. 2. Hoffman, and M. Simic. J. Phys. Chem., 76, 847 (1972). (5) J. Rabani and M.S. Matheson, J. Phys. Chem., 70, 761 (1966). (6) in the case of p-nitrotoluene the spectrum of p-nitrobenzyl radical was not observed because it was masked by an intense spectrum of the nitrotoluene anion radical. (7) W. T. Dixon and R. 0. C. Norman, J. Chem. SOC.,4857 (1964). (8) H. Fischer, Z.Naturforsch. A, 20, 488 (1965). (1) (2) (3) (4)
Substituent Effects on Benzyl Radicals
136'9
TABLE I: Esr Parameters of Substituted Benzyl Radicalsu
Radical
a factor
aCHmH
anH
amH
amH
2.00260
16.34 (2)
5.13 (2)
1.77 (2)
6.17
2.00273
15.76 (2)
5.07 (2)
1.77 (2)
2.00258
16.40 (2)
5.13;5.13
2.00266
16.23;16.07
2.00293
1.75
Other hyperfine constants
6.10
5.07
1.77;1.82
6.14
14.91 (2)
4.63(2)
1.10(2)
2.00253
16.26 (2)
5.06;4.77
1.69
2.00254
16.10 (2)
5.13 (2)
1.78 (2)
2.00273
15.93 (2)
5.06 (2)
1.58 (2)
0.63(CH3)
2.00257
16.07 (2)
5.13 (2)
1.75 (2)
6.70 (CH3)
2.00292
15.25 (2)
4.90 (2)
1.75 (2)
0.54 (CH3)
2.00258
15.64 (2)
5.06 (2)
1.81 (2)
0.92 (N)
2.00283
16.43 (2)
5.26 (2)
1.74 (2)
2.00315
16.08 (2)
5.22 (2)
1.83 (2)
5.92
14.04 (F) 0.50(CI) 0.54
a Produced in irradiated aqueous alkaline solutions, saturated with N20, by the reaction of 0- radicals with the substituted toluenes. Theg factors wore determined by comparison with the signal from the silica cell and are accurate to &0.00003. The hyperfine constants are given in gauss and are accurate to 'r0.03G. The number of protons displaying the splitting is given in parentheses if other than one. Hyperfine constants of 16.4, 5.1, 1.6, and 6.3 G respectively for the CH?,ortho, meta, and para protons were reported by Dixon and Norman (ref 7) and valuesof 16.40, 5.17, 1.77, and 6.19 G by Fischer (ref 8). From ref 4.
CH2C6H4F).'The coupling constants for the ortho protons vary only from 5.06 to 5.26 G and those for the meta protons from 1.55 to 1.83 G. Although most radicals involved substitution a t the para position, the effect on the para proton hyperfine constants in the three ortho- or metasubstituted radicals was similarly very small (5.92 to 6.10 G as compared to 6.17 for the para proton in benzyl). The present results can be contrasted with differences as large as a factor of 2 observed for the nitrogen coupling constants in substituted nitrobenzene radical anions and similar differences found for the methyl protons in substituted phenylpropanesemidiones.2 Hehre, Radom, and Popleg have recently calculated the distributions of electron spin density in a number of substituted benzenes. They find, in general, that in all cases substitution produces changes of less than 10% in the spin density on the ring carbon atoms. Substituents such as CH3, F, COOH, or CN affect the distribution only at the level of -4% or less. From this work one expects only small effects of substitution on the relative values of hyperfine constants of the ring protons of the benzyl radicals since these protons provide a direct probe of the spin density distribution. We find here that, in fact, neither the absolute values of the hyperfine constants of the ring protons nor those of the CH2 protons, which provide a direct measure of the spin
density on the side group, change appreciably. It is seen that substitution has an effect on the benzylic spin system of the magnitude calculated by Hehre, et al., for the sirnple aromatic molecules. There is, however, no quantitative agreement between the predicted effects on spin density and the changes in esr parameters reported here. T!he very large effects noted above for the nitrobenzene anions and the semidiones2 are undoubtedly due to the fact that the spin density distribution in the ?r system is affected appreciably by the spin probe itself so that the effects of substitution are magnified considerably. One can also compare the present results with the relatively large effects of substitution observed in some aliphatic radicals. FischerlO has, for example, interpreted the changes produced by C02H or CN substitution as resulting, respectively, from 7 and 15% withdrawal of electron spin density from adjacent radical sites. In recent work from these laboratories effects of the order of 301% were found to be produced by CN, COzH, or COZ- substitution in the allylic radicals derived by H atom addition to the furan system.11 However, C02- substitution in allyl itself is found to have little effect.4 (9) W. J. Hehre, L. Radom, and J. A. Popie, J. Amer. Chem. SOC.,9 4 , 1496 (1972).
(IO) H. Fischer, 2. Naturforsch. A, 19, 866 (1964). The Journal of Physical Chemistry, Vol. 77, No. 1 1 , 1973
C. Brecher and K. W. French
1370
We have attempted to obtain a correlation of the esr parameters with the substituent constants for the various groups but find no obvious correlation within the small but accurately known differences between the data given in Table I. We can, however, make a few remarks on the details of a number of the individual comparisons. First of all it is seen that the p-methylbenzyl radical has hyperfine constants almost identical with those for benzyl. The 6.70-G constant for the CH3 protons provides a measure of the spin density a t the para position. A comparison with benzyl can be obtained by multiplying the observed value by the ratio of the a and p proton hyperfine constants of the isopropyl radical (22.1 and 24.7 G).12 This gives an equivalent coupling constant of 6.00 G which can be compared with the 6.17-G constant of benzyl. A similar calculation in the case of the cyano derivative based on the 3.51-G nitrogen splitting observed in the CHzCN radical13 would indicate a nitrogen coupling of 1.03 G. The drop in the proton hyperfine constants shows that a small amount of spin density is lost from the ring to the CN group but it is seen that, in fact, a -10% decrease in the nitrogen splitting from that expected is observed. The para fluoro derivative shows a very small increase in the proton hyperfine constants. If we use the aH to aF ratio of 3.97 to 8.41 G observed14 for the nitrobenzene anion and its fluoro derivative, one expects a fluorine hyperfine constant in the P - F C ~ H ~ Cradical H~ of 13.1 G which agrees quite well with the observed value of 14.04 G. A comparison of this latter value to the 6.17 G in the benzyl radical indicates that an a fluorine atom in a planar radical should have a hyperfine constant of -50 G. This value is somewhat lower than the 64 G observed15 for the CH2F radical which is, however, slightly nonplanar as indicated by the 13C hyperfine constant.
The three isomeric carboxybenzyl radicals can be compared internally and with benzyl itself. It is seen that in the ortho and meta derivatives the parameters are essentially identical with those of benzyl. The para isomer, however, shows lower hyperfine constants and a higher g factor indicating a small shift of spin density onto the carboxyl group. Substitution of COz- on the side chain of benzyl radical produces a somewhat larger effect in that it reduces all of the coupling constants by -10% (a,t1 = 15.04, aoH = 4.67, alnH = 1.58, upH = 5.51 G, g = 2.00298).4 The difference between the two isomeric -0CsH4CHz radicals is very much more pronounced. Substitution a t the meta position has essentially no effect on the benzylic system. Substitution at the para position, however, causes a very large decrease in all of the hyperfine constants and a marked increase in the g factor. Both changes indicate pronounced transfer of the electron to the oxygen end of the molecule. In summary, the effects noted here are disappointingly small and do not appear to be able to provide a measure of the substituent effects. They do, however, accurately reflect the fact that substitution causes only small changes in the spin distribution. It is obvious from this that even small changes in spin distribution can have relatively large effects on the reaction rates and ionization equilibria used to measure the substituent constants. R. H. Schuier, G. P. Laroff, and R. W.Fessenden, J. Phys. Chem., 77, 456 (1973). R. W. Fessenden and R. H. Schuler, J. Chem, Phys., 39, 2147 (1 963). R. Livingston and H. Zeldes, J. Magn. Resonance, 1, 169 (1969). A. H. Maki and 0. H. Geske, J. Amer. Chem. Soc., 83, 1852 (1961). R. W. Fessenden and R. H. Schuier, J. Chem. Phys., 43, 2704 (1965).
Spectroscopy and Chemistry of Aprotic Nd3+ Laser Liquids’ C. Brecher” and K. W. French Waltham Research Center, GTE Laboratories Incorporated, Waltham, Massachusetts 02154 (Received January 2. 7973) Publication costs assisted by GTE Laboratories
Measurements have been made on various aspects of the spectroscopy of the SeOClz and aprotic Nd3+ laser liquids, with emphasis on the latter. Unusual features in the chemical behavior are described and some apparent inconsistencies resolved. The formation of complex POzCl2-Lewis acid groups is deemed to be a major factor in the solubilizing of the Nd3+ ion in Poc13 solution. A consistent and COherent model is proposed to explain the observations.
Introduction has advances in the liauid laser oneof the been the develoiment of aprotic hosts-for the Nd3+ ion. This ion, which is the basis for the most widely used class of crystalline and glass lasers, had never before been usable in the liquid state because of its high susceptibility to The Journal of Physical Chemistry, Vol. 77,
No. 11, 1973
nonradiative deexcitation. The importance of high-energy vibrations in this quenching process, and the mechanism (1) This research was partially supported by the Advanced Research Projects Agency of the Department of Defense and was monitored by the Office of Naval Research under Contract No. N00014-68-C0110.
