PHOTOCHEMISTRY OF THE ORTHO-NITROBENZYL SYSTEM IN SOLUTION - EFFECTS OF O-H DISTANCE AND GEOMETRICAL CONSTRAINT ON THE HYDROGEN TRANSFER MECHANISM IN THE EXCITED-STATE

Two rigid nitrobenzene derivatives, 5-nitro-1,2,3,4-tetrahydro-1,4-methanonaphthalene (6), and 5-nitro-1,2,3,4-tetrahydro-1,4-ethanonaphthalene (7), have been synthesized and studied by picosecond flash absorption spectroscopy, steady-state irradiation, and MMX molecular mechanics calculations. Transients with lifetimes of 770 and 410 ps have been detected and assigned to the excited triplet states of 6 and 7, respectively. Consistent with prediction, the omicron-quinonoid intermediate is not detected in the time domain between the laser excitation and the end of decay of the triplet state. Intramolecular abstraction of the bridgehead benzylic hydrogen by the cited triplet state proceeds with relative rates of 1:125 for 6 and 7, respectively. Since structural constraints prohibit the formation of an orthoquinonoid intermediate in these systems, formation of the usual nitroso-alcohol 13 represents the first substantiation of the biradical route to product (3' --> 4, Scheme 1). Absorption of the intermediate triplet biradical resulting from hydrogen abstraction by the triplet excited state was not observed, and is therefore believed to be either very weak in the 400-700 nm region, or located outside the detection region. The kinetic results together with structural parameters determined by molecular mechanics show good correlation between structure and reactivity. Hydrogen atom transfer can proceed at up to 53-degrees deviation from linearity of the C-H-O nuclei, with transfer rates decreasing sharply as the O...H distance increases from 1.3 to 1.6 angstrom. It is suggested that the conditions of the reaction from the excited triplet state meet those required for a hydrogen tunneling mechanism.