ABSOLUTE RATES OF HETEROCYCLIC SINGLET BIRADICAL REACTIONS DETERMINEDBY NANOSECOND TIME-RESOLVED ABSORPTION-SPECTROSCOPY - DIMERIZATIONS AND CYCLOADDITIONS TO ALKENES AND TO DIOXYGEN

Singlet biradicals of the 3,4-dimethyleneheterocycle series, where the heterocycle is furan, thiophene, or N-substituted pyrrole, with chrom ophores in the UV-vis region (560-640 nm), are generated by nanosecond laser flash photolysis of diazene precursors in fluid solution. The r ate constants for disappearance of these transients are determined by time-resolved absorption spectroscopy using the long-wavelength band. The rate constants for dimerization are near the encounter-controlled rate. Rates of trapping by alkenes are well described by a frontier or bital model. Oxygen trapping rate constants range between 10(7) and 10 (9) M-1 s-1. The ratios of rate constants for trapping of 3,4-dimethyl ene-N-(p-toluenesulfonyl)pyrrole by pairs of alkenes in preparative co mpetition experiments are in good agreement with those determined by t he flash photolytic method. This confirms assignments of the 3,4-dimet hyleneheterocycle structure to the chromophore of the biradical transi ents.