Abstract
The highly stable nitroxide radicals 5(6)-chloro-2-(N-tert-butyl-N-aminoxyl)benzimidazole (4) and 5,6-dimethyl-2-(N-tert-butyl-N-aminoxyl)benzimidazole (5) were synthesized and characterized by electron spin resonance spectroscopy, crystallography, and magnetic susceptibility. Both crystallize as dimeric pairs with N−H hydrogen bond donors and N−O acceptors. This behavior contrasts to that of analogous 2-(N-tert-butyl-N-aminoxyl)benzimidazole (3), which forms hydrogen-bonded chains (Ferrer, J. R.; Lahti, P. M.; George, C.; Antorrena, G.; Palacio, F. Chem. Mater. 1999, 11, 2205−2210). Susceptibility analysis shows dimeric antiferromagnetic exchange coupling with J/k = −22 K for 4 and −24 K for 5. Density functional theory (DFT) computations support a N−O to N−O throughspace antiferromagnetic exchange mechanism as the major contributor to the overall magnetic behavior of 4 and 5. A spin−polarization exchange coupling mechanism involving a cyclic exchange path using the −N−H···O−N hydrogen bonds in the dimer...
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