Several trimethylenemethane-type (TMM-type) dinitroxide biradicals have been prepared that differ in tert-butylaminoxylphenyl ring torsion angles by virtue of different steric demands of their "spinprotecting groups". In addition, the synthesis of a TMM-type dinitroxide having a planar π-system is reported. The synthetic route employed for the planar TMM-type biradical is general and should therefore expand the utility of TMM-type biradicals that are susceptible to bond torsions that attenuate exchange coupling. The compounds presented are designed to be effective to map out J-coupling/conformation space in TMM-type biradicals. EPR spectroscopic parameters (hfcc and D-values) were compared to determine if their values reflected differences in conformation in the series of biradicals. Interestingly, neither N-nor H-hfcc varied within our series of biradicals/ monoradicals in a regular fashion, indicating that there is no apparent relationship between N-or H-hfcc and conformation (as judged by molecular mechanics calculations). However, D-values, estimated from relative intensities of ∆m s ) 1 and ∆m s ) 2 transitions, are consistent with a varied degree of delocalization in the dinitroxides: smaller D-values for biradicals having smaller aryl torsions (and greater delocalization), and larger D-values for dinitroxides having greater aryl torsions (and less delocalization). All the biradicals studied, except 5 •• , exhibited linear Curie plots. The linear Curie plots are consistent with both triplet ground-states and singlet-triplet degeneracies. Interestingly, dinitroxide 6 •• exhibited a linear Curie plot, despite the lack of a π-coupling fragment. Biradical 5 •• , however, is a ground-state singlet species with the triplet lying about 140 cal/mol above the singlet.

The syntheses of three new bis(semiquinone)s (2 ••2-, 3 ••2-, and 5 ••2-) linked through carbon−carbon double bonds in a geminal fashion (2 ••2- and 3 ••2-), and through an sp3 carbon (5 ••2-), are presented, as well as the results of variable-temperature EPR (VT-EPR) spectroscopy on these biradicals and two previously reported bis(semiquinone)s, 1 ••2- and 4 ••2-. We suggest that the potential difference in redox couples associated with a biradical is useful for qualitatively assessing changes in the exchange parameter within an isostructural series. The zero-field-splitting parameters for 1 ••2-−5 ••2- are consistent with their electronic structures:  biradicals 1 ••2-−3 ••2- which have conjugating groups attached to the semiquinone rings have D-values less than 5 ••2-, a bis(semiquinone) that lacks such a conjugating group. Also, the D-value of 3 ••2- is significantly less than those of 1 ••2- and 2 ••2-, in agreement with larger interelectron separation in 3 ••2-, a biradical with quinone-methide π-system delocalization. Changes in counterion Lewis acidity are not manifested in the D-values of the biradical dianion 4 ••2-. The EPR spectrum of biradical 5 ••2- is consistent with the existence of at least two rotamers having different zero-field-splitting parameters. Biradicals 1 ••2-−4 ••2- gave linear Curie plots, consistent with J > 0 (ferromagnetic coupling) or J = 0. The temperature-dependent intensity of EPR signals of 5 ••2- are characteristic of antiferromagnetic coupling. Best fit results give J = −114 ± 6 cal/mol for the |D/hc| = 0.01309 cm-1 rotamer of 5 ••2-, and J = −76 ± 3 cal/mol for the |D/hc| = 0.01026 cm-1 rotamer of 5 ••2-.

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The synthesis and characterization of an anion biradical, 1(-)(*)(*), composed of semiquinone and nitroxide functionalities is described. The biradical was prepared by reduction of the nitroxide-orthoquinone precursor using both electrochemical and chemical methods. The zero-field-splitting parameter, |D/hc|, for 1(-)(*)(*) is consistent with the proposed electronic structure, based on a comparison to |D/hc| of related biradicals. Finally, the results of variable-temperature EPR spectroscopy are in agreement with ferromagnetic (high-spin) coupling of the two unpaired electrons through the m-phenylene group.

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