2,3,5,6-TETRAKIS(METHYLENE)-1,4-CYCLOHEXANEDIYL (1,2,4,5-TETRAMETHYLENEBENZENE), A DISJOINT NON-KEKULE SINGLET HYDROCARBON BIRADICAL

The title compound 4a, a purple transient whose main UV-vis absorption band occurs near 490 nm (epsilon = 5000 M-1 cm-1), has been generated by irradiation of 2,3,5,6-tetramethylene-7-oxonorbornane (5a). It has been observed directly by immobilization in frozen matrices or in pol ymer films and by time-resolved spectroscopy following nanosecond late r flash photolysis of the ketone precursor 5a. Although the matrix-imm obilized irradiated samples containing biradical 4a show a triplet ele ctron spin resonance (ESR) spectrum at low temperature, some other spe cies is the carrier, since the ESR spectrum persists even after comple te photobleaching of the UV-vis spectrum characteristic of 4a. The sin glet spin state for biradical 4a, predicted by theory, is confirmed no t only by the absence of an ESR spectrum assignable to it but also by the appearance of a C-13 cross-polarization magic angle spinning NMR s pectrum (delta = 113 ppm) observed when glassy preparations of 5a-2,3- di-(CH2)-C-13 are irradiated. The rates of dimerization of 4a in fluid media, (1.9-3) x 10(10) M-1 s-1, and the rate of capture by O2, 1.86 x 10(7) M-1 s-1, are also consistent with the behavior expected of a s inglet biradical.