The influence of scavenging on CIDNP field dependences in biradicals during the photolysis of large-ring cycloalkanones Original Research Article

Investigation of the CIDNP field dependences of acyl-alkyl biradicals formed by Norrish type-I reaction during the photolysis of cycloundecanone and cyclododecanone in the presence of the scavenger CBrCl3 has been performed at low (up to 0.08 T) magnetic fields. In addition to the emissive polarization with the main maxima caused by the ST− mechanism we observed an absorptive polarization with maxima at low fields (< 0.01 T) for α-CH2 protons of initial ketones. A quantitative analysis of CIDNP amplitude dependences on scavenger concentration allows the estimation of the kinetics of biradical geminate recombination near the emissive maxima. It is demonstrated that the biradical scavenging rate constant can be determined using the kinetics data for high magnetic fields. The scavenging rate constant of CBrCl3 estimated by this method is (2.3 ± 0.4) × 109M−1s−1. Two competitive channels of singlet-triplet conversion have been revealed and the qualitative picture of their contribution to the kinetics of the low-field CIDNP is presented