TEMPERATURE-DEPENDENCE OF THE EXCHANGE COUPLING IN POLYMETHYLENE CHAIN BIRADICALS - CONCLUSIONS REGARDING THE MECHANISM OF THE COUPLING

Exchange interactions, J, in polymethylene biradicals with chain lengths ranging from 8 to 16 carbon atoms, have been measured as a function of temperature in hydrocarbon solvents by time-resolved electron paramagnetic resonance (TREPR) spectroscopy. Fitting the EPR spectra gives accurate J values (+/- 10%), and the chain-length dependence at a particular temperature can be modeled successfully only by using a linear combination of through-bond and through-solvent coupling mechanisms. Temperature-dependent coefficients for each mechanism are extracted from fits of the distance dependence. Through-bond contributions are found to dominate for the C-8 biradicals, while the through-solvent mechanism is the major contributor to J in the C-16 chains. For C-12 and C-14 biradicals, intermediate contributions of the two mechanisms are found. The conformational dependence of the mechanisms and the scope and limits of the model are discussed.