TIME-RESOLVED EPR SPECTROSCOPY IN THE Q-BAND MICROWAVE REGION .2. MAGNETIC-FIELD EFFECTS ON CIDEP IN RADICALS, BIRADICALS, AND SPIN-CORRELATED RADICAL PAIRS

Investigations of chemically induced dynamic electron spin polarizatio n (CIDEP) in free radical, biradical, and spin-correlated radical pair electron paramagnetic resonance (EPR) spectra at two microwave freque ncies are reported. Time-resolved EPR spectra in both organic and aque ous solutions were obtained at X-band (9.5 GHz) and at Q-band (35 GHz) . Comparison of CIDEP patterns at the two frequencies shows that the e xternal magnetic field greatly influences both the shape and the time evolution of the EPR spectra. Hydrogen atom abstraction in the acetone /2-propanol system, and Norrish I alpha-cleavage of several alkyl and benzyl ketones are used to give examples of these unusual effects in d oublet state spectra of radical pairs. Acyl-alkyl and bis(alkyl) birad icals from the cleavage of 2,2,12,12-tetramethylcyclododecanonc show d ramatic magnetic field effects in their EPR spectra, the former from t he large g-factor difference between the radical centers, and the latt er due to field dependent state-mixing of singlet and triplet sub-leve ls. A spin-correlated radical pair, obtained via the abstraction of hy drogen atoms by benzophenone-d10 triplet from the alkyl chain interior of a sodium dodecyl sulfate (SDS) micelle, also exhibits minor magnet ic field effects. Aqueous solution and variable temperature flow TREPR spectroscopy at Q-band are described.