Effect of substituents on spin density in benzimidazole nitronyl nitroxide radicals studied by electron spin resonance.

Several novel benzimidazole-3-oxide-1-oxyl radicals with substituents at 5 and/or 6 position were synthesized. The ESR analysis of nitrogen hyperfine coupling constants (hfccs) revealed that substituents at 5 and 6-position affect the spin density to greater extent than substituents on the phenyl ring at 2-position. Density functional theory calculations of nitrogen hfccs were performed using several different Pople type basis sets, as well as double and triple zeta quality individual gauge for localized orbital (IGLO-II, IGLO-III) and electron paramagnetic resonance (EPR-II, EPR-II) basis sets.

Photolysis and oxidation of azidophenyl-substituted radicals: delocalization in heteroatom-based radicals.

2-(4-Azidophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (14), 2-(4-azidophenyl)benzimidazole-1-oxide-3-oxyl (16), 2-(4-azidophenyl)-1,2,6-triphenylverdazyl (19), 2-(3-azidophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (21), and (3-azidophenyl)-N-tert-butyl-N-aminoxyl (25) were photolyzed in frozen solution to give S = 3/2 state ESR spectra of the corresponding nitrenophenyl radicals with the following zero-field splitting parameters: |D/hc| = 0.277 cm(-1), |E/hc| < or = 0.002 cm(-1) (7 from 14); |D/hc| = 0.