ON THE MANIFESTATION OF S-T-TRANSITIONS IN WEAK MAGNETIC-FIELDS - CIDNP FOR RADICAL PAIRS WITH LARGE HFI CONSTANTS

The CIDNP effects observed in photolytic reactions of (2,4,6-trimethyl benzoyl)diphenylphosphine oxide (TMBDPO) and benzoylphosphonic acid di methyl ester (BDMP) in organic solvents of different viscosities are d etermined by S-T- transitions that occur in the region of energy level s crossing. An analytical expression is proposed for calculation of th e CIDNP effects at weak magnetic fields, which takes into account the S-T- transitions due to energy levels crossing. The experimental data are compared to the results obtained by the numerical solution of the stochastic Liouville equation (SLE) for a microreactor model. The para meters that affect the efficiency of the S-T- transitions are discusse d.