LOW-TEMPERATURE DISAPPEARANCE OF SPIN CORRELATION IN FLEXIBLE 1,N-BIRADICALS (N=22, 24, 26)

The direct detection electron paramagnetic resonance (EPR) spectra of three flexible polymethylene 1,n-biradicals (n = 22, 24, and 26) in te trahydrofuran solution at low temperatures are reported. The acyl-alky l biradicals, which show broad spin-polarized EPR transitions at room temperature and above, exhibit unusual structure at temperatures below 240 K. Simulations show that this additional fine structure is due to a collapse of emissive and absorptive transitions into each other, ca used by a steady decrease in the spin exchange interaction (J) between the unpaired electrons, The effect is more pronounced at lower temper atures as the chains assume more extended conformations. For the C-26 biradical the lowest temperature spectrum clearly shows the residual d oublet state CIDEP (emissive/absorptive) indicative of uncorrelated ra dical centers. This transition from correlated to uncorrelated spins i n a tethered chain is discussed in terms of through-bond and through-s olvent spin-spin coupling mechanisms averaged over the equilibrium dis tribution of chain conformers.