Study of Consecutive Biradicals from 2-Hydroxy-2,12-dimethylcyclododecanone by TR-CIDNP, TREPR, and Laser Flash Photolysis

Several time-resolved spectroscopic techniques (¹H CIDNP, time-resolved EPR, and nanosecond laser flash photolysis) have been used to investigate geminate reactions of consecutive biradicals generated from photolysis of 2-hydroxy-2,12-dimethylcyclododecanone (<BO>1</BO>). It is shown that the primary photochemical process is Norrish type I cleavage, yielding an acyl−ketyl biradical, which is transformed to an alkyl−ketyl biradical via decarbonylation. The rate constant for decarbonylation, estimated by theoretical modeling of the TR-CIDNP kinetics, is (4.2 ± 1.0) × 10⁵ s^-1. The calculations are based on the numerical solution of the stochastic Liouville equation for the Fourier transform of the biradical spin density matrix. The lifetime of the acyl−ketyl biradical (τ = 67 ± 8 ns) has been measured using laser flash photolysis. The distribution of reaction products in the photolysis of <BO>1</BO>, including recombination to the starting ketone, was determined from the CIDNP amplitude and kinetics. The reaction products include unstable compounds containing enol functional groups, which are transformed to the keto form during the dark reaction of tautomerization.