DYNAMIC EFFECTS IN SPIN-CORRELATED RADICAL PAIR THEORY - J-MODULATIONAND A NEW LOOK AT THE PHENOMENON OF ALTERNATING LINE WIDTHS IN THE EPR-SPECTRA OF FLEXIBLE BIRADICALS

Expressions for contributions to T-1 and T-2 from modulation of the ex change interaction (J) in spin-correlated radical pairs (SCRPs) are de rived and incorporated into SCRP theory for simulation of direct detec tion EPR spectra of alkane chain biradicals. The T-2 term is consisten t with that previously derived by Luckhurst to explain the temperature -dependent alternating line widths observed in EPR spectra of stable n itroxide biradicals. Alternating line-width patterns are observed for symmetric bis(alkyl) biradicals at high temperatures, where the rapid modulation of J is caused by conformational jumping. However, in unsym metrical systems such as acyl-alkyl biradicals, J modulation manifests itself at low temperatures as an increased broadening of the spectral lines with increasing distance of the transition from the center of t he spectrum, with no alternation. Several experimental examples are pr esented, simulated, and discussed. Contrary to a suggestion by Maeda e t al., J modulation is shown to have only a minimal effect on T-1 in S CRP theory for the case of biradicals undergoing rapid conformational interconversion. A connection is proposed between the magnitude of the relaxation matrix elements and the biradical conformational distribut ion and chain dynamics.