Diastereomers of paramagnetic Sn semiquinone complexes of type 1 (the P,S isomer is shown on the right) can be distinguished at 213 K by ENDOR spectroscopy; this finding establishes unequivocally the helicity of 4,5-disubstituted phenanthrene semiquinones. The activation barrier for racemization of the phenanthrene moiety (coalescence temperature of 240 K) is appreciably lower that those of the corresponding quinones; it is also strongly dependent on the charge of the complex.
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Mäurer et al. (1991) studied this question.
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