HIGH-FREQUENCY (95 GHZ) EPR SPECTROSCOPY TO CHARACTERIZE SPIN ADDUCTS

EPR spin-trapping experiments are usually carried out at X-band (9.5 GHz) because of the good concentration sensitivity and ready availability of this method. The radical precursors are then characterized from an analysis of isotropic hyperfine coupling and comparison of these coupling factors with those for the reference spin adducts. These experiments encounter two major challenges:  (i) spin adducts from many carbon-centered free radicals have g factors that are nearly the same (resulting in strongly overlapping spectra at 9.5 GHz), and (ii) measurable hyperfine couplings correspond to interactions of the electron spin with just the nearest nuclei. Therefore, very little or no information is obtained on the overall structure of the spin adduct molecule. Some of these difficulties can be overcome by carrying out spin-trapping experiments at 10-fold higher frequency, 95 GHz (W-band). Examples of two spin adducts with nearly the same isotropic g factors (Δgiso = 1.2 × 10-4) are the benzene solutions of ph...