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(11) E. Hayon, Trans. Faraday SOC., 61, 723 (1965). (12) K. Y. Lam and J. W. Hunt, Int. J. Radiat. Phys. Chem., 7, 317 (1975). (13) A. 0. Allen, “The Radiation Chemistry of Water and Aqueous Solutions”, Van Nostrand, Princeton, N.J., 1961. (14) M. Anbar and J. K. Thomas, J. Phys. Chem., 68, 3829 (1964). (15) Cz. Stradowski, unpublished results. (16) H. Taube, Trans. Faraday SOC., 53, 656 (1957). (17) D. Biedenkapp, L. G. Hartshorn, and E.J. Blair, Chem. Phys. Lett., 5,379
(1970). (18) W. A. Chupka, J. Berkowitz, and D. Gutman, J. Chem. Phys., 55, 2724 (1971). (19) E. Abel, Z.Phys. Chem., 136, 161 (1928). (20) H. A. Liebhafsky, J. Am. Chem. SOC., 54, 3499 (1932). (21) J. C. D. Thynne and A. G. Harrison, Trans. Faraday SOC., 62, 2468 (1966). (22) K. R. Ryan, J. Chem. Phys., 52, 6009 (1970).
Spin Trapping of Cyanoalkyl Radicals in the Liquid Phase y Radiolysis of Nitriles S. W. Maot and Larry Kevan* Depafiment of Chemistry, Wayne State University, Detroit, Michigan 48202 (Received April 23, 1976)
The following radicals have been identified in the liquid phase y radiolysis of several nitriles by spin trapping with phenyl tert-butyl nitrone: CH2CN in acetonitrile, H and CH&HCN(?) in propionitrile, CH(CN)2 in malononitrile, and H, CN, and CHzCHzCN in succinonitrile. y proton splittings are observed for the CHzCN and CH(CN)2 spin adducts. The results are discussed in comparison with solid phase radiolysis data and with alkyl radical spin adduct splittings.
Introduction The radicals produced in the y radiolysis of alkyl cyanides (nitriles) have been studied to some extent in the solid phase by electron paramagnetic resonance (EPR), but definite radical identification has proved difficult due to lack of spectral reso1ution.l Here we report liquid phase studies on acetonitrile, propionitrile, malononitrile, and succinonitrile in which the radicals produced by y radiolysis are “spin trapped” by addition to phenyl tert-butyl nitrone (PBN) to form a radical stable in solution.2 PBN is advantageous as a spin trap because it forms a very stable radical adduct, but its main disadvantage is that the spin adduct does not generally show hyperfine couplings from magnetic nuclei in the trapped radical itself. The EPR spectrum of the spin adduct with PBN is generally a triplet of doublets due to splitting by the nitrogen and the p proton. The identification of the trapped radicals is based on small changes in the magnitudes of the nitrogen and p-proton coupling constants of PBN which vary with the size and electronegativity of the trapped radical. Positive identification of the trapped radical is dependent upon synthesis of a series of spin adducts and correlations from the trends deduced. However, for several of the cyanoalkyl radicals reported here, the hyperfine structure of the trapped radical in the P B N spin adduct is resolved. This seems to be related to the electronegativity of the cyano group. Experimental Section The nitriles were obtained from Aldrich Chemical Co. and Fisher Chemical Co. Acetonitrile (CHsCN) and propionitrile (CH~CHZCN) were further purified by repeated distillation. + Current address: Department of Chemistry, T u n g h a i University, Taichung, Taiwan. The Journal of Physical Chemistry, Vol. 80, No. 21, 1976
Malononitrile (NCCH2CN) and succinonitrile (NCCH2CHzCN) were purified by vacuum sublimation. Purified PBN was obtained from Dr. N. A. LeBel of this Department. Samples were prepared by dissolving some P B N into the above compounds. The solution was then degassed under vacuum and sealed in a 2-mm i.d. Spectrosil quartz tube. The y irradiation was usually carried out at room temperature to a typical dose of 0.02 Mrad in a 6oCosource with a dose rate of 4 . 2 3 Mradk. The EPR spectra were obtained with a Varian E-4 spectrometer. After a spin adduct was formed, the solvent could be changed, if desired, by pumping out the original solvent under vacuum and distilling another solvent (typically benzene) into the sample tube. Most spin adducts were found to be very stable; they typically lasted for several days. Nevertheless, fresh samples were prepared for each run, and EPR spectra were taken immediately after y irradiation to avoid complications from slow secondary reactions.
Results Acetonitrile (CH3CN). For a typical dose of 0.02 Mrad an EPR spectrum similar to that shown in Figure 1is obtained for 0.1 M PBN in acetonitrile. The EPR spectrum can be observed after only 0.002 Mrad dose, increases with dose to 0.01 Mrad, and then remains constant a t higher doses. Identical spectral were also obtained for PBN concentrations down to 0.01 M. Although a large concentration of the radical cannot be produced ‘by high irradiation dose, the adduct radical is stable for as long as several days a t room temperature. The six-line triplet of doublets typical of PBN spin adducts is further split into approximately 1:2:1 triplets which are attributed to the hyperfine splitting of two equivalent y protons. The radical trapped by PBN is thus identified as CH2CN. The spin adduct has the following structure:
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Liquid Phase y Radiolysis of Nitriles
5 G
Figure 1. EPR spectrum of the spin adduct in y irradiated 0.1 M phenyl fed-butyl nitrone in acetonitrile (CH3CN)at room temperature at 0.2 Mrad dose in benzene solvent. The spectrum is assigned to the .CH2CN adduct.
although the CHzCH2CN adduct is also quite possible. At lower dose,
