CALIBRATION OF A NEW HOROLOGERY OF FAST RADICAL CLOCKS - RING-OPENING RATES FOR RING-ALKYL-SUBSTITUTED AND ALPHA-ALKYL-SUBSTITUTED CYCLOPROPYLCARBINYL RADICALS AND FOR THE BICYCLO[2.1.0]PENT-2-YL RADICAL

Rate constants have been determined at 37-degrees-C for the ring opening of a variety of alkyl-substituted cyclopropylcarbinyl radical ''clocks'' by nitroxide radical trapping (NRT) using TEMPO. Relative yields of unrearranged and rearranged trialkylhydroxylamines were measured at various TEMPO concentrations, and these data were then combined with absolute rate constants for the reactions of structurally appropriate alkyl radicals with TEMPO as determined by laser flash photolysis. Cyclopropylcarbinyl radicals, including the bicyclo[2.1.0]pent-2-yl radical, were generated by H-atom abstraction from the parent hydrocarbon and, in a few cases, also from the appropriate diacyl peroxide. Twelve substrates yielded sixteen clock reactions because some cyclopropylcarbinyls can undergo two different ring-opening reactions. For six methyl-substituted cyclopropylcarbinyls rate constants ranged from 0.8 x 10(8) s-1 for 1-methylcyclopropylcarbinyl to 47 x 10(8) s-1 for pentamethylcyclopropylcarbinyl. Rate constants for the ring opening of cyclopropylcarbinyl, alpha-methyl- and alpha,alpha-dimethylcyclopropylcarbinyl are 1.2, 0.70, and 0.88 x 10(8) s-1, respectively. Rate constants for H-atom ion by tert-butoxyl from various positions in the 12 cyclopropane substrates relative to the rate of H-atom ion from cyclopentane have also been determined by using competitive NRT.