Communications to the Editor
1881
ICATIONS TO THE EDITOR
Reactivity of Aromatic Compounds toward Positroniuni Atoms"
Sir: Several author^^-^ have analyzed the attack of hydroxyl radicals, hydrated electrons, and hydrogen atoms on aromatic compounds in terms of electrophilic and nucleophilic mechanisms. They correlated the effects of substituents on the reactivity of benzene and benzoate ions toward these species with the appropriate u constants of Hammett's equation for electrophilic and nucleophilic reactions. In view of these investigations it appeared interesting to study the effects of substituents on the reactivity of aromatic compounds toward another simple radical, the positronium atom (Ps),which is the bound state of a positron and an elecixon (ese-). This species, which can be considered as the lightest isotope of hydrogen (in which the proton is replaced by a positron), can carry out substitution, addition, and oxidation reactions and thus behave chemically very much ]like an analog of hydrogen. However, there is a large mass difference between the hydrogen atom and the positronium atom (Ps mass = of H mass). Therefore, one would expect to see differences in the structure of the compounde formed, their formation energies, and their reaction kinetics.5 Benzene and most, of the monosubstituted benzenes are relatively unreactive toward thermal Ps atoms (rate constant IC = lo7A4-l s e ~ - l ) . However, ~?~ nitrobenzene reacts a t a rate of 2.7 X IOIO M-l sec-l. Thus the highly reactive nitrobenzene and nitrobenzene derivatives in very dilute benzene solutions were chosen to assess the effect of substituents attached to the aromatic ring on the reactivity of the compounds toward thermal Ps. Since the observed rate constants for aliphatic nitro compounds are =IO7 M-l sec-l, i.e., of the same order of magnitude as for benzene, it is evident that the combination of an aromatic ring system and a nitro group is responsible for the enhanced reaction rates. All compounds were purified as previously described.7 The 22Na positron source consisted of a thin film of 22NaHC03evaporated on to an aluminum foil (1 mil thick) immersed in the organic solution. The sample was thoroughly degassed using a simple vacuum freeze-thaw technique to remove all dissolved oxygen. Positronium annihilation spectra were obtained by standard techniques.8 By varying the millimolar concentration of the compounds in benzene, rate constants for the reactions of thermalized Ps atoms were obtained. The observed rate constants for the para-substituted nitrobenzenes were analyzed by applying Hammett's equation, 01, the inductive effect, and the four terms for T delocalization effects (uRO, UR(BA), UR-, and
RELATIVE RATE CONSTANTS FOR Ps REACTIONS WITH SUBST. NITROBENZENES vs HAMMETT CONSTANTS F(p,m)
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- 0.75 - 0.50
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1 META SUBST.
0.25
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?(P*m) Figure 1. Log (K/KNB). vs. ti(,,:,,,). K = rate constant far reaction of thermal Ps with substituted nitrobenzene. KNB = rate constant for reaction of thermal Ps with nitrobenzene. (All constants determined in very dilute benzene solutions, < M.) @R+).' As indicated in Figure 1, where log (K/KNB)is plotted us. 8, the best fit is obtained using a blend of ( i and ~ CR+ valueslO with p ~ = * 0.48 and PIP = 0.34. K is the observed rate constant for substituted nitrobenzene; K N Bis the observed rate constant for nitrobenzene, 2.7 (k0.2) X 1O1O M-" sec-l; 6 = ~ R ' U R + pr'ul; u values were taken from ref 9. The rate constants for the corresponding meta compounds can be fitted with Hammett's equation by setting ,A = = 0.35 where p~~ = 0.12 and p~~ = 0.34. This is consistent with the expected trend that resonance effects are of less importance in meta-substituted compounds.9 The analysis does not allow one to determine whether the Ps attack occurs at the aromatic ring or whether the NO2 group is the center of the Ps reaction. However, coupled with an earlier analysis of reaction energeticsll and the experimental results for aromatic compounds containing three NO2 groups,12it does seem probable that the NO2 group is the center for Ps attack. It unambiguously proves, in connection with the previously observed ortho effect,7J3J4 that parameters which govern ordinary chemical reactions also control the chemical reactions of the Ps atom.
+
The Journal of Physical Chemistry, Vol. 78. No. 18. 1974
Communications to the Editor
1882
(1) Work supported by the V . S.Atomic Energy Commission. (2) M. Anbar and E. J. Hart, J. Amer. Chem. SOC.,86, 5633 (1964). (3) (a) M. Anbar, B. M'eyerstein, and P. Neta, J. Phys. Chem., 70, 2660 (1966); (b) Nature (London), 208, 1348 (1966); (c) P. Neta and L. M. Dwfman, J. Phys. C:hem., 73,413 (1969); (d) P. Neta and R. H. Schuler, J. Amer. C b m . Snc., 94, 1056 (1972); (e) P. Neta, Chem. Rev., 72, 533 (19'72). (4) For references on reactions of hydrated electrons with organic compounds see E. J. Hart and M. Anbar, "The Hydrated Electron," Wiley-lnterscience, NEWYork, N. Y., 1970. (5) For e discussion of the properties and interaction of the Ps atom with matter see the following general reviews: (a) J. H. Green and J. Lee, "Positronium Chemistry," Academic Press, New York. N. Y., 1964; (b) V. I. Goldanskii, At. Energy Rev., 6, 3 (1968); (c) J. 0. McGervey in "Positron Acinihilation," A. T. Stewart and L. 0. Roellig, Ed., Academic Press, New York, N. Y., 1967; (d) H. J. Ache, Angew. Chem., Int. Ed. Engl., 11, '179 ('1972); (e) J. H. Green, "Positronium and Mesonic Atoms," in "MTP International Review of Science," Vol. 8, Radiochemistry, A. G. Maddock, Ed., Butterworths, London, 1972; (f) J. A. Merrigan, J. H. Green, and S. J. Tao in "Physical Methods of Chemistry," Vol. I, Part IIIC), A. Weissberger and B. R. Rossiter, Ed., Wiley, New York, N. Y., 1972.,p 501 ff. (6) P. R. Gray, C. F. Cook, and G. P. Sturm, J. Chem. Phys., 48, 1145 (1968). (7) W. J. Madia. A. L. Nichols, and H. J. Ache, J. Chem. Phys., 60, 335 (1974). (8) (a) T. L. Williams and H. J. Ache, J. Chem. Phys., 50, 4493 (1969); (b) L. J. Bartal ;and H. J. Ache, J. Phys. Chem., 77, 2060 (1973). (9) S. Ehrenson, R. T. C. Brownlee, and R. W. Taft, "Physical Organic Chemistry," Vol. 10, A. Streitwieser and R. W. Taft, Ed., Wiley, New York, N, 'V.? 1973, p 1 f:. (10) By using ( T R ( ~ ~ Avalues ) a slightly less favorable fit is obtained with pRP = = 0,19, and p!P,"' = 0.33. 0.71, (11) S. J. Tao and J. M. Green, J. Chem. SOC.A, 408 (1968). (12) V. I. Goldanskii, 0.E. Mogensen, and V. P. Shantarovich, Phys. Lett., 32A, 98 (1970). (13) W. J. Maclia, A. L. Nichols, and H. J. Ache, Appl. Phys., 3, 189 (1974). (14) W. J. Madia, A. L. Nichols, and H. J. Ache, Ber. Bunsenges. Phys. Chem., 78, 179 (7974).
Department of ~ / 7 ~ m i s ~ , ~ ~ Virginia Polytechnic Institute and State 8University bcksburg, Virginia 2406 7
Alan L. Nichols William J. Madia Hans J. Ache*
f?eceivedApril 5, 1974
Methyl Radical-Methanesulfenate Anion Pairs FQrmed by Dissociative Electron Capture in y-irradiated Crystalline Dimethyl-& Publication costs asskPed tiy the U. S. Atomic Energy Commission
Srr: Previous studies2-6 have provided esr evidence for weakly interacting alkyl radical-anion pairs formed by the process of dissociative electron capture (eq 1)in y-irradiated crystalline solids. Hitherto, these intermediate species RX +- e- -+ R . ---X(1) have been observed only in cases where the anion X- is a halide2--4(Cl-, Br-, I-) or a pseudohalide526 (CN-, NCS-). We now report an extension to oxysulfur anions as exemplified by the formation of the methyl radical-methanesulfenate anion pair from dimethyl-& sulfoxide (DMSO-dG) according to
-
MeeSO -i- eMeso----* Me (2) As shown in the upper portion of Figure 1, the esr spec-
The Journal of F'hysical Chemistry. Val. 78 No. 18. 1974
(b)
Figure 1. (a) Esr spectra of y-irradiated DMSO-& before (upper spectrum) and after (lower spectrum) raising the temperature from 77 to ca. 82 K for 50 min. Except for the difforence in sample temperature, the spectra were recorded under identical conditions. (b) Esr spectra of y-irradiated DMSO-& at 77 K recorded under identical spectrometer conditions before (upper spectrum) and after (lower spectrum) exposure to visible light. The irradiation dose was 0.8 Mrads in each case.
trum of crystalline DMSOd8 recorded immediately after y irradiation a t 77 K is composed of a well-resolved multiplet and a broader overlapping resonance at low field. The angular dependence of the spectrum for single-crystal samples revealed that the multiplet spectrum is essentially isotropic whereas the underlying spectrum is highly anisotropic. A t low microwave powers (4mW) and certain orientations, the narrow multiplet becomes the dominating feature and consists of a septet pattern with intensity ratios (1:3:67:6:3:1) characteristic of coupling to three equivalent 2H nuclei. Although the septet spectrum is clearly associated with a CD3 group, the isotropic 2H hfs of 2.99 (f0.05) G is appreciably smaller than the corresponding hfs of 3.576 G for the free CD? radical in the liquid state.7 Thus, bearing in mind that the isotropic g factor of 2.0028 is a typical value for alkyl radicals (giso for C D r is 2.002567), the esr results are consistent with a methyl radical which has a spin density p~~ (ca. 0.84) of less than unity in the carbon 2p orbital, as observed in the case ( p = 0.90) of the methyl radical-bromide ion air.^-^ Additional evidence which bears on the nature of the CD3 species in y-irradiated D M S O - ~ G was obtained from thermal annealing and photobleaching studies. As illustrated in Figure 1, the septet esr spectrum disappeared in less than 1 hr when the sample temperature was raised from 77 to ca. 82 K; at 77 K the decay was much slower and required 17 hr for completion. In each case the change was accompanied by the loss of the intense violet color characteristic of a freshly irradiated sample. Optical studies using thin cells showed that the absorption spectrum recorded immediately after y irradiation at 77 K consists of a strong
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