An electron nuclear double resonance study in a glassy matrix of nitroxide radicals with delocalized spin density.

Indolinone (1), benzimidazole (2) and quinoline (3) nitroxide radicals have the nitroxide function in a conjugated position with respect to the π system. Their electron nuclear double resonance (ENDOR) spectra in a [2H8] toluene glassy matrix have been detected and the principal values and the orientation of the proton hyperfine coupling tensors have been obtained by comparison with computer-simulated spectra. Typical features of ENDOR spectroscopy in frozen solution, and in particular its more general feasibility with respect to use in solution, are discussed.