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

EPR spin-trapping experiments are usually carried out at X-band (9.5 G Hz) because of the good concentration sensitivity and ready availabili ty of this method. The radical precursors are then characterized from an analysis of isotropic hyperfine coupling and comparison of these co upling factors with those for the reference spin adducts. These experi ments encounter two major challenges: (i) spin adducts from many carbo n-centered free radicals have g factors that are nearly the same (resu lting in strongly overlapping spectra at 9.5 GHz), and (ii) measurable hyperfine couplings correspond to interactions of the electron spin w ith 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 e xperiments at 10-fold higher frequency, 95 GHz (W-band). Examples of t wo spin adducts with nearly the same isotropic g factors (Delta g(iso) = 1.2 x 10(-4)) are the benzene solutions of phenyl and trichlorometh yl adducts of phenyl tert-butylnitrone (PEN). It is shown that, for a mixture of these spin adducts, the spectra from two species are resolv ed at W-band whereas the spectral lines severely overlap at X-band, Fo r these spin adducts, additional line broadening at 95 GHz caused by a n enhanced contribution from rotational modulation of the electronic g matrix is much too small to offset the gain in resolution due to Delt a g. It also is shown that parameters of rotational diffusion can be u sed to characterize spin adducts, even those with very similar local m olecular structures and almost identical magnetic parameters, such as methyl-, ethyl-, and tetradecyl-PBN. These parameters are more accurat ely measured at W-band and characterize the spin adduct molecule as a whole. Multifrequency EPR data for toluene solutions of methyl- and te tradecyl-PBN show that the rotational diffusion of methyl-PEN is aniso tropic with rho(x) = 2.7 +/- 0.3 and rho(y) = 3.6 +/- 0.3, while the r otation of tetradecyl-PBN is essentially isotropic with rho(x) x rho(y ) = 1.0 +/- 0.20. The last indicates that the long alkyl chain of tetr adecyl-PBN in solution is likely to be positioned around the nitroxide moiety, giving the molecule an effectively isotropic motion. Simulati ons of W-band EPR spectra from spin adducts with resolved proton hyper fine structure and analysis of motional data for these compounds in th e absence of reliable data on anisotropic proton hyperfine couplings a re also discussed.