Kishan Bhatia and Robert H. Schuler
1888 (3) N. P. Yao and D. N. Bennion, J. Phys. Chem., 75, 1727 (1971). (4) N. P. Yao and D. N. Bennion, J. Electrochem. Soc., 118, 1097
(1971). (5) D. E. Arrington and E. Griswold, J. Phys. Chem., 74, 123 (1970). (6) M. Della Monica and U. Lamanna, J. Phys. Chem., 72, 4329
(1968).
(7) D. F. Evans, C. Zawoyski, and R. L Kay, J. Phys. Chem., 69, 3878
(1965).
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(8) C. Trelner and R. M. Fuoss, Z. Phys. Chem., 228, 343 (1965). (9) A. C. Harkness and . M. Daggett, Jr., Can. J. Chem., 43, 1215
(1965). (10) W. A. Adams and K. J. Laidler, Can. J. Chem., 46, 2005 (1968). (11) D. F. Evans, J. Thomas, J. A. Nadas, and . A. Matesich, J. Phys. Chem., 75, 1714 (1971). (12) S. R. C. Hughes and D. H. Price, J. Chem. Soc. A, 1093 (1967). (13) R. L. Kay, C. Zawoyski, and D. F. Evans, J. Phys. Chem., 69, 4208 (1965). (14) D. F. Evans and P. Gardam, J. Phys. Chem., 72, 3281 (1968). (15) D. F. Evans and P. Gardam, J. Phys. Chem., 73, 158 (1969). (16) B. J. Barker and J. A. Caruso, J. Amer. Chem. Soc., 93, 1341 (1971). (17) R. L Kay, J. Amer. Chem. Soc., 82, 2099 (1960). J. L. Hawes and R. L. Kay, J. Phys. Chem., 69,2420 (1965). (18) (19) (a) T. Shedlovsky, J. Franklin Inst., 225, 739 (1938); (b) R. M. Fuoss and T. Shedlovsky, J. Amer. Chem. Soc., 71, 1496 (1949). (20) R. M. Fuoss and F. Accascina, “Electrolytic Conductance,” Interscience, New York, N. Y., 1959. (21) R, M. Fuoss and L. Onsager, J. Phys. Chem., 61,668 (1957). (22) R. M. Fuoss and E. Hlrsch, J. Amer. Chem. Soc., 82, 1013 (1960). (23) Reference 20, p 242. (24) R. M. Fuoss, J. Amer. Chem. Soc., 79, 3301 (1957). (25) G. Jones and . Dole, J. Amer. Chem. Soc., 51,2950 (1929).
(26) (a) H. Falkenhagen and M. Dole, Phys. Z„ 30, 611 (1929); (b) H. Falkenhagen and E. L Vernon, ibid., 33, 140 (1932). (27) H. S. Harned and B. B. Owen, “The Physical Chemistry of Electrolytic Solutions," 3rd ed, Reinhold, New York, N. Y., 1958, p 240. (28) The viscosity and conductance data of the salts studied in tetramethylurea will appear following these pages in the microfilm edition of this volume of the journal. Single copies may be obtained from the Journals Department, American Chemical Society, 1155 Sixteenth St., N.W., Washington, D. C. 20036. Remit check or money order for $3.00 for photocopy or $2.00 for microfiche, referring to code number JPC-73-1884. (29) R. L. Kay, T. Vltuccio, C. Zawoyski, and D. F. Evans, J. Phys. Chem., 70,2336 (1966). (30) M. Kaminsky, Disc. Faraday Soc., 24,175 (1957). (31) J. P. Bare and J. F. Skinner, J. Phys. Chem., 76, 434 (1972). (32) D. F. T. Tuan and R. M. Fuoss, J. Phys. Chem., 67, 1343 (1963). (33) P. Bruno and M. Della Monica, J. Phys. Chem., 76,1049 (1972). (34) . A. Matesich, J. A. Nadas, and D. F. Evans, J. Phys. Chem., 74, 4568 (1970). Larger values of the ion size parameter (and larger Ka values) for the salts in TMU would have been obtained If the data had been evaluated by the procedures of Justice or by the Fernandez-Prini expanded form of the Fuoss-Hsia and the Pitts equations. In these relatively recent methods of data evaluation a C3/2 term is included In the conductance equations and the ion size parameter is given a new interpretation. (35) Reference 20, p 197. (36) . A. Copian and R. M. Fuoss, J. Phys. Chem., 68, 1177 (1964). (37) G. R. Lester, T. A. Cover, and P. G. Sears, J. Phys. Chem., 60, 1076 (1956). (38) E. D. Wilhoit and P. G. Sears, Trans. Ky. Acad. Sci., 17, 123
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Uracilyl Radical Production in the Radiolysis of Aqueous Solutions of the 5-Halouracils1 Kishan Bhatia and Robert H. Schuler* Radiation Research Laboratories, Center for Special Studies and Department of Chemistry, Mellon Institute of Science, Carnegie-Mellon University, Pittsburgh, Pennsylvania 15213 (Received March 19, 1973)
Publication costs assisted by the
U. S.
Atomic Energy Commission and Carnegie-Mellon University
The radiolysis of dilute aqueous solutions of 5-bromo-, 5-chloro-, and 5-fluorouracil has been investigated by liquid chromatographic and by pulse radiolysis conductometric methods with attention being focussed on determining the importance of the production of 5-uracilyl radical as an intermediate. In the chromatographic study both the consumption of the starting material and formation of uracil have been examined in the presence of a suitable H atom donor. It is concluded that at pH 7 the reaction of hydrated electrons with 5-bromo- and 5-chlorouracil yields 5-uracilyl radical essentially quantitatively. From the conductometric study it is apparent that the loss of chloride from the anion radical initially produced by reaction of eaq_ with 5-chlorouracil is sufficiently slow that in acidic solutions protonation is important and as a result 5-uracilyl radical is produced only in reduced yield. It is estimated that in this case protonation competes with dissociation on equal terms at pH 5.2. For 5-bromouracil protonation of the intermediate anion radical is unimportant even at pH 3 indicating that in this case the anion intermediate has a lifetime 10 reaction is 100% efficient. Auxiliary results are presented on other aspects of the radiolysis of the halouracils.
The radiation chemistry of aqueous solutions of 5-bromouracil (BrUr) has been the subject of a large number of investigations because of the importance of this compound as a sensitizer in radiobiological studies.2 Comparative The Journal of Physical Chemistry, Vol. 77. No. 75. 1973
studies of the radiolysis of 5-bromouracil and of 5-chloro(ClUr) and 5-fluorouracil (FUr) have recently been carried out3 and it appears that the rate constants for the reaction of H, OH, and eaq~ with these compounds are known
Radiolysis of Aqueous Solutions of the 5-Halouracils
1889
sufficiently accurately36 that effects of processes which compete with the initial reactions can be taken into account quite well. It has also been known for some time that attack of eaq- on 5-bromouracil produces a large yield of bromide ion4·5 and it has been suggested4 that 5uracilyl radical (Ur·; referred to in the following as uracilyl radical) is formed by reaction 1. Measurements of ha0 I
O
x
xsx
!
Hx + eaq-
II
I
studies show that in acidic solutions of 5-chlorouracil, protonation of the radical anion initially produced competes with its dissociation so that uracilyl radical is produced only in reduced yield. With 5-bromouracil protonation appears to be unimportant down to pH values below 4. Auxiliary measurements on a number of other aspects of the radiolysis of the 5-halouracil systems are also reported.
Experimental Section
.X
XN/XC/
—>
0V
(la)
I
I
\. c /
/ -52
H XUr”
H
XUr O II
XUr"
—
