| Abstract
| - A nitronyl-nitroxide (NIT) biradical D-NIT2 linked by a single double bond has been engineeredand investigated in the solid state by a combination of X-ray diffraction, magnetic susceptibility measurement,EPR, as well as solid-state 1H and 13C NMR spectroscopies, and experimental electron density distribution.All techniques reveal that a double bond is a very efficient coupling unit for exchange interactions betweentwo radical moieties. Using a Bleaney−Bowers model dimer (H = −JS1S2), a singlet−triplet energy gap ofJ = −460 K was found with the singlet state being the ground state. This very strong intramolecularinteraction was confirmed by EPR measurements in CH2Cl2 solution (6 10-4 M) or dispersed in a polymermatrix at low concentration. In keeping with these unusual interactions, solid-state NMR signals of thebiradical were found to be considerably less shifted than those found for related monoradicals. Temperature-dependent solid-state 13C NMR spectra of D-NIT2 confirmed the very strong intramolecular coupling constant(J = −504 K). The electron density distribution of D-NIT2 was measured by high resolution X-ray diffraction,which also revealed that this biradical is an ideally conjugated system. The in-depth characterization includesthe deformation maps and the observed electron density ellipticities, which exhibit a pronounced σ−πcharacter of the ONCCNO cores in keeping with an efficient electronic delocalization along thealkene spacer.
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