ZERO-FIELD SPLITTING VERSUS INTERELECTRONIC DISTANCE IN TRIPLET ELECTRON-SPIN-RESONANCE SPECTRA OF LOCALIZED DINITRENES

Triplet zero-field splitting parameters were obtained in 2-methyltetrahydrofuran glass at 77 K for phenylene-1,4-dinitrene 1), (biphenyl-4,4'-dinitrene (2), (E)-1,2-bis(4'-nitrenophenyl)ethene (3), 1,4-bis(4'-nitrenophenyl)buta-1,3-diene (4) and 1,8-bis(4'-nitrenophenyl)octa-1,3,5,7-tetraene (5). The results were (1) \D/hc\ = 0.169 cm-1, \E/hc\ = 0.004 cm-1 (2) \D/hc\ = 0.189 cm-1, \E/hc\ = 0.00 cm-1, (3) \D/hc\ = 0.122 cm-1, \E/hc\ = 0.00 cm-1, (4) \D/hc\ = 0.0865 cm-1, \E/hc\ = 0.00 cm-1 and (5) \D/hc\ = 0.0442 cm-1, \E/hc\ = 0.00 cm-1. All these biradicals are ground-state singlets. Based on the observed decrease in triplet signal intensities as temperature decreases. The substantial magnitudes of \D/hc\ for 3-5, despite the large distance between localized nitrene electrons, is much more than can be explained by a simple dipolar interaction between localized electrons, and is attributed at least partly to spin polarization effects on the pi-electron clouds of these systems.