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

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Spin trapping experiments are usually carried out with continuous-wave (cw) X-band (9.5 GHz) EPR spectroscopy because of the good concentration sensitivity and ready availability of this method. Characterization of the spin adducts is usually based on 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 close g-values (resulting in strongly overlapping spectra), 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. In this presentation we demonstrate how high frequency EPR at 95 GHz (W-band) with enhanced g-value resolution and sensitivity to sotational motion can be applied to solve some of these problems. Ten-fold better g-value resol ution allows us, e.g., to clearly separate methyl and trichloromethyl phenyl-tert-butylnitrone (PBN) from a mixture at W-band while the presence of two species is practically unnoticeable in the corresponding X-band EPR spectra. Rotational dynamics parameters can be measured much more accurately at W-band, providing information on rotational radii which characterize the adduct molecule as a whole.