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 (Δg iso = 1.2 × 10-4) are the benzene solutions of phenyl and trichloromethyl adducts of phenyl tert-butylnitrone (PBN). It is shown that, for a mixture of these spin adducts, the spectra from two species are resolved at W-band whereas the spectral lines severely overlap at X-band. For these spin adducts, additional line broadening at 95 GHz caused by an enhanced contribution from rotational modulation of the electronic g matrix is much too small to offset the gain in resolution due to Δg. It also is shown that parameters of rotational diffusion can be used to characterize spin adducts, even those with very similar local molecular structures and almost identical magnetic parameters, such as methyl-, ethyl-, and tetradecyl-PBN. These parameters are more accurately measured at W-band and characterize the spin adduct molecule as a whole. Multifrequency EPR data for toluene solutions of methyl- and tetradecyl-PBN show that the rotational diffusion of methyl-PBN is anisotropic with ρ x = 2.7 ± 0.3 and ρ y = 3.6 ± 0.3, while the rotation of tetradecyl-PBN is essentially isotropic with ρ x ≈ ρ y = 1.0 ± 0.20. The last indicates that the long alkyl chain of tetradecyl-PBN in solution is likely to be positioned around the nitroxide moiety, giving the molecule an effectively isotropic motion. Simulations of W-band EPR spectra from spin adducts with resolved proton hyperfine structure and analysis of motional data for these compounds in the absence of reliable data on anisotropic proton hyperfine couplings are also di