V. J. KUCK,E. WASSERMAN, AND Mi. 4.YAGIER
3570
Acknowledgments. We are grateful to Professor Fred Lytle for his generous assistance throughout this program and to the National Science Foundation for support of this research.
shown to be made up of temperature-dependent and temperature-independent components with the temperature-dependent component in benzene and its derivatives attributed to ‘%hermally activated Sl-SO internal conversion via an isomeric state.”2o If this view is valid, increasing a substitution may then be facilitating such isornerisni, and in this light, it is interesting that the sterically crowded pentamethyl- and hexamethylbenzenes also have characteristically high k d values and tcmwratm‘e dependencies for $,re2’
(20) Reference 7, p 140. (21) Upon completion of this work and manuscript, P Communication appeared containing fluorescence data for several polymethylbenzenes. Particularly interesting is the reported observation (from unpublished data) of a large temperature effect on the fluorescence intensity of hexamethylbenzene; cf. A. Reises and 11.J. Leyshon, J . Chem. Phys., 56, 1011 (1972).
Electron Spin Resonance of Triplet Nitrenes from Aryl Isocyanates1 y V. J . Kuck,* E. Wasserman, and W. A. Yager Bell Luborutories, Murray Hill, New Jersey
07974
(Received J u l y 10,1979)
Publication costs assisted by Bell Laboratories
The photolysis of benzoyl azide at 77°K with 2537-A light in a methylcyclohexane-isopentane glass was studied by means of uv and esr. The rearrangement to form phenyl isocyanate was observed in the uv. No triplet signal in the esr corresponding to benzoylnitrene was obtained. However, upon further irradiation a signal was found whose zero-field parameters, D and E , were identical with those of phenylnitrene which had been
independently produced from the photodecomposition of phenyl azide. Phenyl isocoysnate,o-tolyl isocyanate, and naphthyl-1 isocyanate were irradiated at 77°K in a rigid matrix with 2537-A light, and the zero-field pararntters of the nitrenes that were produced were found to be the same as the parameters of nitrenes made from the corresponding azides. No nitrene signals were detected by means of esr upon irradiation at877 and 4”E of alkyl isocyanates with and without sensitizers.
We have observed the electron spin resonance (esr) spectra of aromatic nitrenes obtained from the photolytic decomposition of aromatic isocyanates. It had been previously postulated that isocyanates yielded nitrenes upon photolysis2 although no direct evidence had been obtained supporting this conclusion. While this work was in progress, Porter and Ward4 reported the observation of the ultraviolet spectrum of phenylnitrene during the flash photolysis of phenyl isocyanate. Initially we sought to clarify the existence of triplet benzoylnitrene as an intermediate in the photolytic decomposition of benzoyl azide. A 1% solution of benzoyl azide in 95% methylcyclohexane-5~o isopenta5e was prepared and then irradiated at 77°K with 2537-A light. A Varian V-4502 1QO-Hz field modulation esr spectrometer was used to detect resonance absorptions. i h esr spectrum was obtained which was identical with one observed when phenyl azide was photolytically decomposed to form ~henylnitrene.~ KO lines other than those corresponding to phenylnitrene were detected. Thus, there was no direct evidence for t r plet hcnzoylnitrene as an intermediate in the photolyric decomposition of benzoyl azidc. The Journal of P h p i c a l Chemistry, Vol. 75, X o . 24, 19‘72
I n another experiment, the photolytic decomposition of benzoyl azide in the same matrix and under the same conditions was observed by ultraviolet spectroscopy. Absorption lines at 269 and 277 nm, corresponding to the formation of phenyl isocyanate, appeared upon irradiation. The decomposition of the phenyl isocyanate was then thought to be the source of phenylnitrene. A 1% solution of phenyl isocyanate in methylcyclohexane-isopentane was irradiated a t 77°K and the decomposition was followed by means of esr. The observed spectrum was identical with that recorded when phenylnitrene had been generated from phenyl azide. These results might be explained by the following equation.
(1) Given in part a t the Middle Atlantic Regional Meeting, Feb 1968, held in Philadelphia, Pa., by V. J. Kuck. (2) (a) D. A. Bamford and C. H. Bamford, J. Chem. Soc., 30 (1941); (b) J. H. Boyer, W. E. Krueger, and G. J. Mikol, J . Amer. Chem. SOC.,89,5504 (1967). (3) J. S.Swenton, Tetrahedron Lett., 2855 (1967). (4) G. Porter and B. Ward, Proc. Roy. SOC.,Sec. A , 303, 139 (1968). (5) G. Smolinsky, E. Wasserman, and W. A. Yager, d . Amer. Chem. Soc., 84,3220 (1962).
3571
DNA BASECATIONRADICALS PRODUCED BY PHOTOIONIZATION O
11
hv
CeHb---CN3 ---+
r 0 - l /I -+CeH6NC0-% (n--Cn.
j
CBH6-N
*
+CsH&"
Other 1y9solutions of aromatic isocyanates in methylcyclohexane-isopentane were irradiated at 77°K and the esr spectra of the resulting nitrenes were recorded. From the spectra the zero-field parameters D and E6 were determined, where D is a measure of the magnitude of the spin-spin interaction of the unpaired elec, its deviation from cylindrical symmetry. trons and @ Assuming g := 2.0823 the following values were obtained : phenylnibrene ID/ = 1.003 cm-l, 0-tolyl nitrene ID1 := 0.957 and naphthyl-1-nitrene \Dl = 0.798; IEl < 0.003 in each case. These results agreed with those previoiisly obtained when the corresponding azide wab decomposed.7 Previously, by iise of esr, it has been established that alkyl nitrenes are generated in the photolytic decomposition of alkyl azides.8 If the decomposition was carried out in the presence of sensitizers, nitrene forma/tion could be observed by means of esr at 77°K. When direct irradiation of alkyl azides was employed there
was much less efficient generation of stabilized nitrenes and a signal could be detected only a t 4°K. I n hope of obeerving alkyl nitrenes generated from the photolytic decomposition of alkyl isocyanates, 1yo solutions of cyclohexyl, tert-butyl, and n-propyl isocyanates were irradiated with an Osram HBO 200-W mercury lamp both a t 77 and 4°K. No esr absorptions corresponding to nitrene formation were detected. These results could be explained by the fact that alkyl isocyanates absorb between 2100 and 2400 AIzaa region where our lamp output is very low. Sensitizers such as acetone, benzene, and acetophenone were used and again no signals corresponding to nitrene formation were observed. The failure of sensitized decomposition might be due to inefficient energy transfer to the excited triplet states of the alkyl isocyanate. Since population of tlie excited singlet states occurs far into the uv, the nearby triplet states may well be of high energy. (6) E. Wasserman, L. C. Snyder, and W. A. Yager, 1. Chem. Phys., 41, 1763 (1964). (7) E. Wasserman, Progr. Phys. Org. Chem., 8, 319 (1971). (8) E. Wasserman, G. Smolinksy, and W. A. Yager, 3. Amer. Chem. Soc., 86,3166 (1964).
Spin Resonance Study of Several DNA Base Cation Radicals Produced by Photoionization by M. D. SeviIla,* C. Van Paemel, and C. Nichols Department of Chemistry, Oakland University, Rochester, Michigan
@OS$
(Received M a y 16, 1971)
Publication costa assisted by Oakland University
a-Cation radicals of a number of DNA bases have been produced in aqueous glasses (8 N NaOD, 5 &fK2COaD3P04) at low temperature by uv photolysis. Well-resolved esr spectra of wcations of 5-methylcytosine (NaOD, K&Oa, D3P04), thymine (K~COS), and thymidine (NaOD) are found and reduced to hyperfine coupling constants. The methyl proton splittings for n-cations of 5-methylcytosine show a great dependence on p1-I. The methyl proton splitting is 20 G in 5 M K&Oz and 5.7 M DaP04, whereas in 8 N NaOD it is 14 G. This lowered splitting is explained by a comparison of the experimental results with theoretical splittings based on McLachlan ?VI0computations. This comparison suggests that at high pH the amino nitrogen loses one proton and fully conjugates with the ring n-electron system. Results found for thymine in 5 M K2C03 are nearly identical with previous results in NaOD and DaP04. The thymidine a-cation is found to have similar splittiiigs to the thymine n-cation, however, anomalously low g values are found for this radical. D28, 5.7
Introduction Positive and negative ions of the DNA bases are likely primary intermediates in the radiolysis of crystalline pyrimidines and purines as well as in the radiolysis of Dn'A itself, Recent esr investigations by several
authors have implicated ions as the primary intermediates toward radical production in the radiolysis of pyrimidine single crystals.*-4 Several workers have (1)J. HBttermann, J. F. Ward, and L. s, Myers, Phys. Chem.. 3,117 (1971).
J ,
Radiat.
The Journal of Physical Chemist~u,Vol. 76, N o . 1.4,1971
