CHAIN DYNAMICS CAUSE THE DISAPPEARANCE OF SPIN-CORRELATED RADICAL PAIR POLARIZATION IN FLEXIBLE BIRADICALS

X-band time-resolved electron paramagnetic resonance (TREPR) spectra o f 1,16- and 1,21- acylalkyl biradicals,; obtained in toluene solution at low temperatures (187-253 K), are reported. The spectra show a stro ng temperature dependence in their patterns of chemically induced elec tron spin polarization.; The spin-correlated radical pair (SCRP) mecha nism dominates at high temperatures, while the radical pair mechanism (RPM) is the main pattern at lower temperatures. It is postulated that the SCRP spectrum is suppressed at the lower temperature due to molec ular motion on a time scale which modulates the exchange interaction ( J) between the unpaired electrons. Simultaneously the RPM is enhanced by this motion. Average values of J calculated from end-to-end distanc e distributions for the C-21 biradical show that in the absence of thi s dynamic effect, an intense SCRP spectrum would be present at the low er temperature. The results demonstrate that the interpretation of ave rage J couplings may be difficult in temperature regions where these m otional effects dominate the appearance of the spectrum.