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Photoinduced Isomerization of Ion Radicals
splitting and the anisotropic dipolar tensor. Nevertheless, the total splitting with the field along the axis of the 2p orbital can be separated into isotropic and dipolar contributions by the following approach.” For the dipolar contribution, values of 20 range form 103 to 120 G,12with the former value usually perferred.13 The isotropic splitting is due most probably to spin polarization and can be estimated from the relationship a. = -41pzp, which has been shown to hold rather well for ?r radi~a1s.l~ Thus we can write All = 4103 + 41)p2, = -144~2,. Applying this to the observed All = 99 G, we obtain pZp= 0.69. The isotropic splitting thus calculated is -28 G. This value is close to the value of -24 G, obtained from data of ref 4, with the following choice of signs: All = -103 G, A 1 = +15 G. Since the total spin density on 0-is less than 1 (0.69), the implication is that a considerable amount of spin is delocalized onto the surrounding matrix of 0-in alkaline ice glass. Acknowledgment. This research was partially supported by the U.S. Energy Research and Development Admin-
istration under Contract No. E(11-1)-2086.
References and Notes L. Kevan in “Actions Chimiques et Biologlques des Radiations”, Vol. 13, M. Haissinksy, Ed., Masson, Paris, 1969, pp 57-84; L. Kevan in “Radiation Chemistry of Aqueous Systems ’, 0. Stein, Ed., Wiley-Interscience, New York, N.Y., 1968, pp 21-71. J. H. Lunsford, Catal. Rev., 8, 135 (1973). J. R. Brailsford and J. R. Morton, J. Chem. Phys ., 51, 4794 (1969); J. R. Brailsford, J. R. Morton, and L. E. Vannotti, J. Chem. Phys., 49, 2237 (1968). Y. Ben Taartt and J. H. Lunsford, Chem. Phys. Left., 19, 348 (1973), and references therein. S. Schllck, P. A. Narayana, and L. Kevan, J. Chem. Phys., 64, 3153 (1976). N.-B. Wong and J. H. Lunsford, J. Chem. Phys., 55, 3007 (1971). A. Carrington and A. D. McLachlan, “Introduction to Magnetic Resonance”, Harper and Row, New York, N.Y., 1967, pp 132-138. P.H. Kasai, J. Chem. Phys., 43, 3322 (1965). J. Rabani, Adv. Chem. Ser., No. 81, 131 (1968). B. Segall, G. W. Ludwig, H. H. Woodbury, and P. D. Johnson, Phys. Rev., 128, 76 (1962). E. Melamud, S.Schiick, and B. L. Silver, J. Magn. Reson., 14, 104 (1974). P. B. Ayscough, “Electron Spln Resonance in Chemistry”, Methuen, London, 1967, Appendix 111. S. Schlick, J . Chem. Phys., 56, 654 (1972). E. Melamud and B. L. Silver, J. Phys. Chem., 77, 1896 (1973).
Photoinduced Isomerization of Ion Radicals. The Conversion from 1,&Cyclohexadiene to 1,3,5-Hexatriene Cation Radicals Tadamasa Shlda,” Tatsuhlsa Kato, and Yoshlo Nosaka Department of Chemistry, Faculty of Science, Kyoto Unlvefsity, Kyoto 606, Japan (Received January 13, 1977)
The cation radical of 1,3-cyclohexadiene isomerizes to the cation radical of all-trans-1,3,5-hexatrieneupon photoexcitation with visible light. The conversion involves three intermediate cations of hexatriene having the conformations of cis-cis-cis, cis-trans-cis, and trans-cis-trans. All the cations of the triene exhibit similar but distinguishable electronic absorption spectra in the visible and the near-IR regions. The conformation of the cations as well as the photoconversion among them are consistently accounted for by INDO and CNDO/S calculations for the cations. Some kinetic analysis is made for two conformers in the photostationary state.
Introduction Although the photochemical ring opening of 1,3cyclohexadieneto 1,3,5-hexatrieneis a familiar electrocyclic process with abundant documentations,’ nothing is known about the behavior of the respective cation radicals under photoexcitation. In this work the cation radicals were newly produced in rigid matrices at low temperatures and the photoinduced conversion was studied by optical absorption spectroscopy. The absorption bands of the cation radicals appear in a convenient spectral region from the near-infrared through near-UV, which permits detailed studies on the wavelength dependence of the photoinduced conversion. As a consequence of the present study the cation radical of 1,3-~yclohexadieneis found to convert ultimately to the cation radical of aZl-trans-1,3,5-hexatriene upon illumination with light of X 400-500 nm: L
A400-500 nm.-IOO%
The above conversion, however, is not a single reaction,
Scheme I
0 A
+ /=
”\==/--=_/c’ D
E
but involves three intermediate cationic species as shown in Scheme I. The intermediates B, C, and D are very sensitive to light and change as the arrows indicate. The electronic spectra of the four conformers B, C, D, and E are similar but are distinct enough to permit optical studies on the individual steps in the scheme. The potential surfaces calculated by the method of INDO and CNDO/S are consistent with the proposed scheme and account for The Journal of Physlcal Chemistry, Vol. 81, No. 11, 1977
T. Shida, T. Kato, and Y. Nosaka
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TABLE I: Excited States of Cation Radicals of Cyclohexadiene and Hexatriene Transition Oscillator Maior configurations Molecule State energv. eV strength 1,3-Cyclohe~adiene~lZB, 2.04 (2.72)b 0.0304 (0.0208) 0.832 (0.850) ( n I 1 n z 2 )+ 0.503 (0.492) ( n l 2 n j 1 ) 2'B, 4.04 (3.53) 0.4452 (0.1318) 0.513 (0.518) ( n I 1 n z 2 + ) 0.837 (-0.804) ( n I 2 n j 1 ) (A) 1,3,5-Hexatriene lzBg 1.70 (2.3oj 0.0845 (0.0301) 0.852 (0.784) ( n 1 2 n 2 1 ~ 3 20.427 ) (0.518) ( T ~ ~ T ~ ~ w (t-t-t) (E) 2,Bg 2.87 (3.99) 0.7433 (0.6900) 0.438 (0.566) ( n l Z ~ z 1 n 3t20.785 ) (0.772) ( n 1 2 n 2 z n 4 1 ) 1,3,5-Hexatriene 12A, 1.70 (2.19) 0.0446 (0.0129) 0.859 (0.812) ( n l Z n z 1 n j 2+) 0.415 (0.481) (n1znz2n41) 12B, 2.98 (3.87) 0.0259 (0.0182) (t-c-t) (D) 0.807 (0.687) ( T ~ ~ T +~0.343 ~ T (-0.570) ~ ~ ) ( ~ ~ ~ n ~ (cis band) +0.276 (-0.413) ( T ~ ~ T ~ ~ ~ ~ ~ T ~ ~ ) 2,A, 2.92 (3.89) 0.6163 (0.5420) 0.425 (-0.528) ( R ~ ~ t ~0.803 ~ (0.802) ~ T ~( T ~ ~) ~ ? T ~ 1,3,5-Hexatriene lZBg 1.68 (2.16) 0.0799 (0.0149) 0.856 (-0.729) ( T ~ ~ - 0.424 ~ ~ (0.597) ~ T ~ ( n 1~2 n 2) z ~ 4 1 ) (c-t-c) (C) 2'B, 2.86 (3.62) 0.4993 (0.4270) 0.435 (0.632) ( n 1 2 n 2 1 n 3 2+) 0.801 (0.740) ( n 1 2 n 2 2 n 4 1 ) I I
I
The structure of the cation radical of 1,3-~yclohexadienewas assumed to be the same as that of neutral molecule determined by Oberhammer and Bauer." The geometrical parameters for all hexatrienes were assumed as C-C 1.46 A , C=C 1.35 A , C-H 1.10 A ,
