95-670 GHz EPR studies of canthaxanthin radical cation stabilized on a silica-alumina surface

The 95-670 GHz EPR measurements at 5 K were performed on canthaxanthin radical cation chemically generated on silica-alumina. The 327 GHz and higher frequency EPR spectra were resolved into two principal components of the g tensor. Spectral simulation indicated this to be the result of 8 anisotropy where g(parallel to) 2.0032 and g(perpendicular to) = 2.0023. This type of g tensor is consistent with the theory for polyacene pi-radical cations, which states that the g tensor becomes cylindrically symmetric with increasing chain length. This also demonstrates that the symmetrical unresolved EPR line at 9 GHz is due to a carotenoid pi-radical cation with electron density distributed throughout the whole chain as predicted by RHF-INDO/SP molecular orbital calculations. The lack of temperature dependence of the EPR line widths over the range of 5-80 K at 327 GHz suggests rapid rotation of methyl groups even at 5 K that averages out the proton couplings from three oriented beta-protons. In fact, similar line widths at 5 K were observed at 670 GHz. Simulation of EPR spectra at 95-250 GHz gives only symmetrical unresolved lines. The present work shows that the 327-670 GHz EPR measurements are sufficient to resolve the individual g tensors of C-H containing pi-radicals in powder and frozen glasses. Symmetry differences can be deduced from which radical identification can be made.