J. LAMBORN AND A. J. SWALLOW
920
Vol. 65
THE PItOTECTION EFFECT IN THE y-RADIOLYSIS OF BRXZENECYCLOHEXANE RIIIXTURES AND ITS EXPLANATION IS TERMS OF SELECTIVITY OF THE PRIMARY RADIATION ACT BY J. LAMBORN AND A. J. SWALLOW Nuclear Technology Laboratory, Department of Chemical Engineering and Chemical Technology, Imperial College, London, S.W. 7 , England Recezued September 7, 1960
Polyrner yields in r-irradiated benzene-cyclohexane mistures are lower than would be obtained if the radiolysis of the two components proceeded independently, this being in agreement with previous messurements of hydrogen yields. The results are explained in terms of preferential excitation of the ?r electrons of benzene by the fast electrons produced in the system.
Introduction The yield of hydrogen in irradiated cyclohexanebenzene mixtures is very much less than would be obtained if the radiolysis of the two components proceeded independent1y.l The decrease in yield is partly due to the removal of hydrogen atoms produced from cyclohexane, by the reaction2
milturc before irradiation, and the total dose given, the Gvalncl for conversion to polymer was obtained.
Results and Discussion
The results of our work are shown in Fig. 2. In the case of pure cyclohexane the yield for cyclohexane converted to polymer (G = 4.03) is in excellent agreement with other determinations of the same q ~ a n t i t y , gand , ~ with the value of G = 2.0 H. + Ci“ -3- C6H7. --+ polymer (1) for dicyclohexyl (ie., G = 4.0 for cyclohexane conbut the major part of the decrease appears to be verted) in the fast-electron irradiation of cyclodue to “protection” of cyclohexane by benzene, hexane.6s10 It should be noted that although the possibly by some form of energy transfer. Al- nature of the polymer depends on dose rate, though hydrogen is the major irradiation product of dicyclohexyl being the only product a t high dose cyclohexane, polymer is almost equally important. rates6 whereas cyclohexylcyclohexene appears at, In the case of benzene, polymer is by far the major lower dose rates,g the yield of total polymer would product.3 It therefore seemed desirable to seek not be expected to depend appreciably on dose confirmation of the protection effect by examining rate. This is confirmed by the present work. the irradiated mixture for polymer. I n the case of benzene our yield of G = 0.98 is in good agreement with G = 0.93 as determined for Experimental The irradiation vessel was a glass tube sealed by two y-rays by Gordon, et a1.,8 and does not differ mercury traps (Fig. 1). The vessel was cleaned, dried and appreciably from G = 0.75 as determined for fast weighed, and filled with the misture to be irradiated. electrons by Patrick and Burton. “Spectrosol” cyclohexane and “Analar” benzene from We have compared our polymer yields with yields Hopkin and ’PJilliams Ltd. were used. Mercury was added calculated from Burton and Patrick’s determinaand the system was deaerated by the passage of nitrogen (oxygen-free) which had been saturated by the organic tions of hydrogen yield.2 To do the calculation mixture’s vapor by passage through a gas washing bottle. we have assumed that the hydrogen in the mixture The system was irradiated with y r a y s from a kilocurie is formed partly by unimolecular decomposition of cobalt-60 ~ o u r c e . ~The dose rate was determined with the the two constituents and partly by reactions of Fricke dosimeter (in 0.1 N H2SOa).G being taken as 15.5.6 hydrogen atoms such as The dose rate in the dosimeter was 0.72 X 10“ rad./hr. The dose rate in the mivture was obtained by multiplying H. CsHn -3- Hz CtHii. (2) the dose rate in the Fricke dosimeter by the ratio of the
+
electron densities of the mixture and water. Mixtures were given a total dose of 40-70 X lo6 rad. and after irradiation the vessel and contents were weighed. Volatile substances were evaporated off by the passage of nitrogen while the system was heated in a boiling water-bath. The vessel and contents, (mercury polymer), were then weighed to constant weight. This procedure gave an accurate result since neither cvcloheuane6 nor benzene,7 nor presumably a mixture of the two, gives a significant amount of material of molecular weight intermediate between the original and about twice the original moleciilar weight. After weighing, the mercury was separated from the polymer and weighed. Thus by difference the amount of high boiling product was determined. Knowing thr weight and composition of the
+
(1) J. P. Manion and M.Burton, J . Phys. C h e m , 66, 560 (1952). (2) M.Burton and W. N. Patrick, zbid., 8 8 , 421 (1954). (3) W. N. Patrick and M . Burton, J . Am. Chem. Soc.. 76, 2626 (1954). (4) G. R. Hall and M .Streat. t o be poblkhed. ( 5 ) J. L. Haybittle. R. D. Saunders and A. J. Swallow, J . Chem. Phys., 26, 1213 (1956). (6) 1%.A. Dewhurst, J . Phys. Chem., 65, 813 (1959). (7) S. Gordon, A. R. Van Dyken and T. F. Doumani, ibid., 62, 20 (1958).
H* $. CeHs -3- H?
+ + CG&.
(3)
The hydrogen yield in the mixture is then given by G H =~ d H~ t b
+ (1 - u)G&c
(4)
where GH~,,,is the yield of hydrogen as corrected for reaction 1 by Burton and Patrick,2 GH2b and G H are ~ the yields of hydrogen from pure benzene and pure cyclohexane, respectively, and “a” is thP fraction of energy absorbed by the benzene. Othei mechanisms may contribute to the formation of hydrogen, but would not be likely to make an appreciable difference to equation 4. Similarly we have assumed that polymer is formed exclusively by combination of the radiation-produced organic free radicals with each other, and that no organic radicals react in any other may. (8) P. J. Horner and A. J. Swallow, zbzd., 65, 953 (1961). (9) E. S. Waight and P. Walker. J . Chem. Snc., 2225 (1960). (10) T. D Nevitt and L. P. Remsberg, J Phys. C h e r n , 64, 969 (1960).
PROTECTIOS EFFECT IN 7-RADIOLYSIS OF BENZENE-CYCLOHEXANE
June, 1961
92 1
The yield for monomer converted to polymer in thc mixture, Gpm, is then given by Gpm
= UGpb
+ (1 - U)Gm
(5)
where G p b and Gpc are the yields of monomer converted to polymer for pure benzene and pure cyclohexane, respectively. If disproportionation is an important reaction of free radicals or if polymer is formed by any other mechanism, then it is possible (but not certain) that equation 5 may need to be modified to some extent. Prom equations 4 and 5 we obtain
(6)
Values of Gpb and GPOmay be taken from our work to be 0.98 and 4.03, respectively. There is no general agreement as to the value of GH?~,but if we are to make use of Burton and Patrick’s other data it seems best to adopt their value of 4.3.2 GHzb may be taken to be 0.036.l Substituting these values in equation 6 we obtain G,,
= 0.715GH2m
-I- 0.96
B
I
(7 )
Polymer yields calculated from this equation are shown in Fig. 2 and are seen to be in reasonable agreement with our experimentally determined Fig. 1.-Irradiation vessel: A, mercury seal pots; B, mixture t o be irradiated. values. It may be that Burton and Patrick’s relatively low value of G H = ~ ~4.3 is due to the presence of impurities in the cyclohexane used. These would have little effect on GHom. If this is the case and we adopt the more generally assumed value of 5.3 for G H ? ~then , for 5% electron fraction benzene (4.7% volume fraction) the calculated polymer yield will be 15% less than shown in Fig. 2. Other calculated points will be in error to a smaller extent. It is clear both from our results and from Burton and Patrick’s that benzene “protects” cyclohexane. To explain this phenomenon we must examine the manner in which the radiation energy is distributed between the two components. The fast electrons produced by Compton scattering cause by far the greatest part of the chemical effect and are often considered to impart energy to the two components approximately in the ratio of their electron fraction in the mixture, irrespective of molecular structure.ll An impression of the irrelevance of molecular structure is also given by stopping power measurements, which show that the stopping power of molecules to fast charged particles is very nearly an additive function of the numbers of atoms present.12 Stopping power however does not provide a sensitive indication of the influence of molecular structure on the transfer of 100 50 100 the energy of the fast electrons to molecules,13 and existing measurements leave it possible that C6H12 CsHe Volume 70. molecular structure may have an important effect. Fig. 2.--Number of molecules of mixture converted to That molecular structure is in fact important is polymer per 100 e.v. as a function of composition by volshown by previous which shows that ume: a, results of this work; X, results calculated from I
(11) M. Burton, W. €1. Hamill and J. L. Magee, Gsneva Con/., 29, 391 (1958). (12) L. H. Gray, Proc. Camb. Phil. Soe., 40, 72 (1944). (13) Cf. R. H. Plataman, “Symposium on Radiobiology,” Ed. Nickson. John Wiley and Sons, New York, N. Y., 1952, p. 139. (14) E. Fermi, Z. Phgszk, 29, 315 (1924). (15) H. A. Bethe, Ann. P h y s i k . 5, 325 (1930). (16) E. Pi. Lassettre, Radialzon Research, S u p p l . 1, 530 (1959).
Burton and Patrick’s work. The line is theoretical, calculated according to equations 8 and 5.
the probability of excitation or ionization by fast electrons is related to the probability of excitation or ionization by electromagnetic radiation. For the present system, the extinction co-
F. P.DELGRWO.iw J. W. GRYDER
922
efficient of benzene is greater than that of cyclohexane throughout the entire range down to 1500 A.,I7 so that benzene will receive more energy than indicated by its electron fraction. An estimate of the energy distribution may be made if we accept the conclusions of Inokuti.ls Inokuti has shown that for the action of fast electrons on benzene the cross section for excitation of a-electrons to low levels is very large, and is about ten times as great as the total inelastic crosssection for u-electrons. Assuming that the cross section per molecule for a-electrons is in the ratio of 24 to 36 for benzene and cyclohexane, and that the picture is not very different for slow electrons, we obtain for the fraction of the total energy taken UD bv the benzene ~”
a =
+
3n( lOEr Eu) 3Eu n(20E7r - Eu)
+
where n = mole fraction of benzene in the mixture, E , = mean energy given to a benzene molecule when it is excited to one of its lowest strongly allowed excited states and E , = mean energy given to a molecule of benzene or cyclohexane when the r-electrons are activated by radiation. We take E , as 7 e.v. since this is between the energy of the first strongly allowed excited state of benzene (6.74 e.v.) and the ionization potential (9.2 e.v.). We take E , as 10 e.v. (the final conclusion is not highly sensitive to the values taken). (17) L. K. Pickett, &I. LIuntz and E. hI. McPherson, J . Am. Chem. Soc., 73,4862 (1951).
(18) hI. Inokuti, Isotopes and Radzation (Tokyo), 1, 82 (1958).
Vol. 65
The fraction “a” can then be determined for various volume percentages, and knowing this, in conjunction with equation 5 , the polymer yield may be calculated. A line calculated on this basis is shown in Fig. 2 . Considering the various assumptions made, the agreement with experiment is remarkably good. We may therefore conclude that the protective action of benzene is due to its preferentially taking up the energy of the fast electrons rather than to an initially random absorption of energy followed by some form of “energy transfer” from cyclohexane to benzene. Although the selective action of sub-excitation electrons has previously been r e c o g n i ~ e d , l the ~-~~ implications of the selectivity of the action of fast electrons do not appear to have been fully discussed and its seems likely that many other cases of “protection” or “energy transfer,”21,22 may be largely explicable in such terms. It should also be noted that selectivity should be particularly important in biological systems, since aromatic groups form a large part of cell constituents such as nucleic acids and proteins. Acknowledgments.-The authors wish to thank Dr. J. Murre11 for helpful comments and the Department of Scientific and Industrial Research for a Studentship granted to one of them (J.L.). (19) R H. Platzman, Radzatzon Research, 2 , 1 (1935). (20) J. Weiss, Nature, 174, 78 (1954). (21) hl Magat, L. Bouby, A. Chapiro and S . Gislon. 2. Elektrochem., 6 2 , 307 (1958). (22) D. R. Kalkwarf, X’ucleonzcs, 18,No. 5 , 7fi 11960).
INFRA4REDAND RAMAN SPECTRAL STUDY OF NITRATE SOLUTIONS IX LIQUID HYDROGEN FLUORIDE‘ BY F. P. DELGRECOAND J. W. GRYDER Department of Chemistry, The Johns Hopkins University, Baltimore, M d . Received September 21 1060 ~
Infrared and Raman spectra are reported for H F solutions of ” 0 8 , X206, KNO,, K F and H20. Comparison of these spectra with those for other systems indicates that nitrates react with anhydrous H F to form the species X02+, HSOI, H 3 0 +and solvated F-. No evidence was found for the species H2N03+ which had been postulated to explain the results of cryoscopic and electrical conductivity measurements.
Fredenhagen interpreted electrical conductivity and boiling point elevation data by postulating that K N 0 3 reacts wit’h liquid anhydrous hydrogen fluoride in accord wit’hthe equation2 2HF
+ KN03 = K + + H2N03’ + 2F-
Since in this reaction “ 0 3 assumes the unlikely role of an acid substantially weaker than HF, the present investigation was undertaken to see if spectroscopic evidence could be found for the existence of H2N03+ in liquid hydrogen fluoride solut’ionsof KXOa and other nitrates. (1) Based on a dissertation to be submitted by F. P. Del Greco to the Faculty of Philosophy of The Johns Hopkins University in partial fulfillment of the requirements for the degree of Doctor of Philosophy. (2) K. Fredenhagen, Z. Elektrochem., 37, 684 (1931),and references cited therein.
Experimental Chemicals.-Liquid hydrogen fluoride was distilled from tanks of General Chemical “high purity” anhydrous H F and used without further purification. Electrical conductivity measurements on this material showed it to have a specific conductance of about 10 X ohm-’ cm.?, and its infrared spectrum was not noticeably different from that obtained for carefully purified H F by Maybury, Gordon and kat^.^ Potassium nitrate \vas twice recrystallized, vacuum oven dried, and stored and dispensed in a drybox. Anhydrous nitric acid was obtained by low pressure room temperature distillation from a solution of T\‘aNOs in a fourfold excess of H2S04 and collected at Dry Ice temperature. Dinitrogen pentoxide was prepared by dehydration of 1007, nitric acid with P,Olo followed bv low pressure room temperature distillation of the product from the mixture and collected at Dry Ice temperature. Bnhv13) R. H. Maybury, S. Gordon and J. J. Katz, J . Chem. P h y s . , 23 1277 (1955).
