Theoretical and expeirmental studies of CIDNP kinetics in recombination of radical pairs by the method of switched external magnetic field. III. Free radicals in homogeneous solution

摘要

The method of chemically induced dynamic nuclear polarization in a switched external magnetic field (SEMF CIDNP) is applied here for the first time to an experimental study of short-lived neutral radicals in homogeneous solutions. With three photochemical reactions it is examplified, that SEMF CIDNP allows investigations of the kinetics of the transient species with high time-resolution as well as a determination of their spin relaxation times in low magnetic fields. A theoretical approach is developed, which permits simulation and analysis of the experimental data. In weak magnetic fields (0.5-2.0 mT) the effective spin-lattice relaxation times for the decay of the chemically induced spin polarizations in benzyl, tert-butyl, and 2-hydroxy-2-propyl radicals are found to be T_1 = (3.8 (+-) 0.5) mus, T_ 1 = (7.8 (+-) 0.5) mus, and T_1 = (2.5 (+-) 0.5) mus, respectively, in benene solution at room temperature. They are in fair agreement with relaxation times determined by time-resolved X-band electron paramagnetic resonance spectroscopy at strong magnetic fields (approx = 350 mT).