Spin and molecular dynamics of biradicals as studied by low field nuclear polarization at variable temperature

H-1 and C-13 chemically induced dynamic nuclear polarization (CIDNP) and ti me-resolved stimulated nuclear polarization (SNP) have been applied to the investigation of spin and molecular dynamics in biradicals, generated in th e photolysis of 2,2,12,12-tetramethylcyclododecanone, at low magnetic field in the temperature range from 200 to 360 K. This reaction involves two con secutive biradical stages, the primary acyl-alkyl and the secondary bis-alk yl biradical, with different reaction products. Characteristic differences in the CIDNP field dependencies, and SNP decay times for nuclei of differen t reaction products are seen at high temperatures. Upon cooling, the emissi on maximum position shifts to low field and the width of the H-1 CIDNP fiel d dependencies of the acyl-alkyl biradical products decreases; Below 243 K these effects reverse direction, furthermore, a significant slowing down of the SNP decay is observed. Calculations of the H-1 and C-13 CIDNP field de pendencies and the SNP kinetics for the primary biradical have been perform ed based on the numerical solution of the stochastic Liouville equation for the biradical spin density matrix. They use a model of restricted diffusio n for the description of the molecular dynamics and take into account the d istance dependence of the exchange interaction. Good quantitative agreement between the calculations and experimental data is found for the whole temp erature range when using the appropriate dependence of some of the paramete rs on temperature. In particular, it was necessary for the quantitative sim ulation in the;low temperature range to introduce a temperature dependence of the biradical reactivity. The model calculations qualitatively confirm t hat the low-temperature broadening of the CIDNP field dependencies, and the increase of the SNP decay time are caused by the slowing down of the molec ular mobility and the decreasing of reactivity. For the products of the sec ondary biradical, a qualitative interpretation is given showing that in com parison with the products of the primary biradical spin-orbit interaction i s of less importance for the biradical spin dynamics, that the influence of the molecular mobility under the temperature variation is more pronounced, that the lifetime is longer, and that the nuclear polarization is more eff iciently formed.