(26) I". A. Cotton and T, (1962).
G. Dunne, J. Amer. Chem. SOG.,84, 2013
(27) C . J. Ballliarasen and C . 1Z. Jylrgensen, Acto. ('hem. Seand., Q, 397 (195.5)
(28) A. 'l'oenita a n d 'v. Tamai, ,I. Phgs. Chem, 75, 648 (1971). (29) 13. iE4uroya and R. Tsuehida, Bull. C h m . Soc. J a p . , IS, 429 (1940).
onance Evidence for Dissociative Electron Capture i
e
L. Kerr, Kathleen Webster, and Ffrancon Williams"
Deparfment of Chemiatrg, Universitg of Tennessee, Knoxville, Tennessee 57916
(Received February 33,1978)
Alkyl radicals are formed during y irradiation of alkyl phosphates a t 77OK. Inclusion of an electron scavenger virtually eliminates this reaction, and a comparable yield of radicals derived from the scavenger is obtained. This proves that alkyl radicals are formed only by dissociative electron capture in the pure esters. Possible biological implications are discussed.
The meohan sin1 or interaction of ionizing radiation with pliosphmk mterrj i s of interest as these compounds are closely related to many important constituents of bialogical matthrials. The effect of y irradiation on alkyl phosph?al,cs liss been investigated by product a.anlysis,2and more recently by esr examination of the Although ~~ radical intermetlistes att Pow t ~ m p e r a t u r e s . ~ /'he Journal of Physical Chentislr:4, Vol. 76,No. 20,1972
various reaction steps have been postulated on Ihe basis of the information obtained, definitive proof of Lhese i s (1) This work was supported by the U. S. iltomie Energy CQUJmission under Contract No. AT-(40-1)-2968, and this is AEC Document No. ORO-2968-72. (2) R. W. Wilkinso? and F. Williams, J . C'hem. Soe., 4098 (1961). (3) S. Sugimoto, K. Kuwata, S. Ohnishi, and I. Nitta, R e p . J a p . Assoc., Rad. Res. Polymers, 7, 199 (1965-1966).
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~
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S
S ELECTRON ~ ~ ~ CAPTURE I A T ~IN ~PHOSPH.4TE ~
2849
ESTERS
hacking. Here we present the results of an esr study which demonstrates conclusively that the alkyl radicals observed in ~ ~ ~ ~alkyl ~ phosphates r a ~ ~are~formed a ~ by dissociath e ~ ~ i : c ~capture. r o ~ ~ This finding conf rms a previous suggestion along these lines.4
Trimethyl phosphate, triethyl phosphate, dimethyl mdhyt ~ h o ~ ~ and ~ dimethyl ~ o ~ aacid~ pyrophosphate e ~ were supplied Ecy Aldrich Chemical Co. and used without purification. Garxipies of each compound were prepared both with. and 1vithou.t a quantity of a known s ~ a - v e ~ ethyl g ~ r ~bromide ~ being used as the ~~Icctrron ~ c ~ forvthe ~nncthyl n esters ~ ~and ~methyl bromide for the ethyl ester. The alkyl halide concentration was nominally t niol XIor greater. It was found necessary l o uw low. ethyl bromide concentrations (1-2 mol %) io avoid forWanziAion of the C2W4Br radical since its q m h x r n tended t o obscure that of the ethyl radical. All samples formed @,lameswhen quenched to 77°K. The doped and unclopec! samples of each compound u rthe e same total were ~ r ~ a ~ at i ~thrs , t e~ e~m ~ e r ~ tLo dose (69.1 to 0 2 I'kilrad), and their esr spectra recorded ihanedial ely u ~ i r i gidentical spectrometer conditions. 'The esr spectrometcc izrid accessories have been describrd elsewhere
In ail the methyl esters that were examined, the quartet ew spectrum of the methyl radical (A,,,CHa = 23 G) was sulwimposed on a broad triplet which can be assigned to a >P(===O)OCPI~I. radical! On the other hand, the somewhat anisotropic 12-line spectrum obtained in the sample doped with ethyl bromide appeared to be mainly due to the ethyl radical = 27 6, A6% = 21 G), indicating that a large fraction of the electrons bad been captured by the ethyl bromide. Representative iqxctra obtained for trimethyl phosphate arc shown in Figures 1A and B, with stick diagrams indicating the spectral analyses.8 Aithough o are quite different, the the spectra of the t ~ radicals positions of the four lines in the methyl radical spectrum lie close tct those of lines 3, 5 , 8, and 10 of the ethyl radical spectrum, thus rendering a relatively small amount of methyl radical difficult to detect in the presence of el hyl. However, the methyl radicals disappeared at 7'a'"K so that any contribution to the spectrum in the doped sample could be determined by recording the spectrum again after the decay was observed t o be complete in the spectrum of the pure sample (Figure IC). The corresponding spectrum of the is shown in Figure 1D. Comparison of dopea Figures d D allows that the intensities of lines 3 and 10 relative t q the adjacent outer lines (which are due only t o the ethyl radical) are diminished in Figure ID. The difference, however, represents only a small contribution to the initial spectrum by methyl radicals,
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I
I
Figure 1. Esr spectra of y-irradiated trimethyl phosphate with and without ethyl bromide. The radiation dose was 0.2 Mrad. All spectra were recorded a t 77°K under the same spectrometer conditions. The spectra refer i o samples as follows: A, pure trimethyl phosphate immediately after y irradiation; B, trimethyl phosphate containing 1-2 mol yo ethyl bromide immediately after y irradiation; C, same sample as A, 6 days after irradiation; and D, same sample as B, 6 days after irradiation.
and scavenging is probably better than 90% complete. Results similar to the above were obtained for the phosphonate and acid pyrophosphate. I n the case of the triethyl phosphate, ethyl radicals were observed in the pure material and methyl in the doped sample. As the ethyl radical signal was almost unchanged after 6 days whereas the methyl had decayed out leaving no trace of ethyl, scavenging by methyl bromide was judged to be complete. Since complete scavenging was observed for triethyl phosphate containing -8 mol yo of methyl bromide, it is reasonable to assume that the incomplete scavenging occurring in the methyl esters was due to the much lower (1-2 mol %) scavenger concentration used (4) A. Begum, S. Subramanian, aud M. C. R. Symons, J , Chem.
SOC. A, 133% (1970). (5) (a) J. M. Warman, K.-D. Asmus, and R.. H. Schuler, Advan. Chem. Ser., No. 88, 25, (1968); (b) E. D. Sprague and F. Williams, J . Chew,. Phys., 54, 5425 (1971). (6) J. Lin, K. Tsuji, and F. Williams, J . Amer. Chem. Sobc., 90, 2766 (1968). (7) Since the yield of the (CHaO)nP(===O)O@H~~ radical from trimethyl phosphate is considerably smaller in the sample doped with ethyl bromide than in the undoped sample (Figure 1) I it; is very probable that this radical originates largely from the decay of methyl radicals by hydrogen atom abstraction from the trimethyl phosphate. For other examples of hydrogen atom abstraction by methyl radicals at 77OX, see (a) E. D. Sprague and F. Williams, J . Amer. Chem. Soc., 93, 787 (1971); (b) R. J. b e Roy, E. D. Sprague, and F. Williams, J . Phys. Chem., 76, 546 (1972) : (c) J.-T. Wang and F. Williams, J. Amer. Chem. Soc., 94, 2930 (1972); (d) A. Campion and I?. Williams, ibid., in press. (8) I n the powder spectrum of the ethyl radical (Figure lD), the signal heights of the sharp lines (2, 5 , 8, and 11) are enhanced relative to the heights of the other lines a5 compared t o the intensity ratios indicated in the stick diagram. This is due t o a line width difference which arises from anisotropic hyperfine broadening. For a detailed explanation, see E. L. Cochran, F. J. Adrian, and V. A. Bowers, J . Chem. Phya., 34, 1161 (1961). The Journal of Physical Chemistru, VoL 76, No. 20, io72
~ ~ t it ~d~ eloar o Lt o us~ that ~ the ~ results reported liere provide definitive evidence for the formation of parent alkyl ritdicnls from the phosphate esters by dissociativcl electron capture, this interpretation has been 1 one of the reviewers of this paper. Therefore some elsboration seems to be necessary. If we assumc that the parent alkyl radicals are not iormed by ciiswciative electron capture, then the prescnce s i an eiicctron scavenger should not affect their
%he Journal of Physical Chemistry, Vol. 76, No. 20, 1978
