FTIR SPECTROSCOPY OF UV-GENERATED QUINONE RADICALS - EVIDENCE FOR AN INTRAMOLECULAR HYDROGEN-ATOM TRANSFER IN UBIQUINONE, NAPHTHOQUINONE, AND PLASTOQUINONE

FTIR spectroscopy has been used to study the generation of radicals at low temperature upon UV irradiation of ubiquinones, naphthoquinones, and plastoquinones. Radical formation was ascertained by EPR spectrosc opy. The effects of solvent, temperature, and chemical nature of the c hain substituent on the radical photogeneration were studied. The pres ence of a side chain bearing at least one isoprene unit plays an essen tial role in the radical generation process. O-18 labeling on both car bonyls and specific C-13 labeling on each carbonyl group were used to investigate the nature of the radicals. UV irradiation at low temperat ure of quinones bearing an unsaturated side chain leads to the formati on of the protonated semiquinone radical. On the basis of FTIR and Ram an data, the C - O stretching mode of the protonated semiquinone radic al of ubiquinone, naphthoquinone, and plastoquinone is observed at app roximate to 1518, 1536, and 1504 cm(-1), respectively. An intramolecul ar hydrogen atom transfer between the first isoprene unit of the side chain and the proximal carbonyl of the quinone ring is demonstrated, s trongly favoring a diradical formation mechanism previously proposed t o occur upon UV irradiation of plastoquinone at room temperature (Cree d, D.; Hales, B. J.; Porter, G. Proc. R. Sec. London, A 1973, 334, 505 ).