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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.

Polarizable radical sites in distonic radical anions are stabilized by ostensibly remote negative charges. Computational evidence suggests bond dissociation energies of closed-shell precursors are significantly lowered by through-space interactions with a proximate negative charge, however direct experimental confirmation has proved challenging. Herein, we exploit two complementary tandem mass spectrometry strategies to probe the influence of a remote charge on the stability of nitroxyl radicals, and vice versa. Dissociation of negatively charge-tagged alkoxyamines reveals that the energetic onset of radical formation is dependent on the proximity and basicity of the charged group, thus providing direct evidence for a charge-induced stabilization of the nitroxyl radical. Complementary kinetic method experiments on a series of proton-bound dimers demonstrate that the presence of a nitroxyl radical decreases the proton affinity for a selection of proximate ionic groups. These data show excellent agreement with quantum-chemical calculations and provide a general framework to explore the magnitude and direction of charge-radical interactions through systematic exploration of the identity, polarity and the proximity of the ion to the radical site. These findings expand our fundamental understanding of radical ion ener-getics that underpin the application of distonic ions as models for neutral radical reactivity, and open new avenues for exploiting these interactions as chemical switches. (C) 2018 Elsevier B.V. All rights reserved.

ChemBioChem, 14 (14), ISSN:1439-4227, ISSN:1439-7633

The spin polarization of excited triplet states caused by mutual annihilation of triplet states was detected by time-resolved electron paramagnetic resonance (EPR) of triplet excitons in two molecular crystals, anthracene-tetracyanobenzene and phenazine-tetracyanoquinodimenthane. The time profile of the two EPR lines of the triplet exciton spectrum have been studied in the time range up to 200 μs after a laser pulse. Besides the initial polarization of the lines, due to intersystem crossing, a long-lasting polarization process is detected which is attributed to the triplet-triplet annihilation. The investigation of the dependence of the phenomenon on the intensity of the light pulse and on the orientation of the crystals in the magnetic field is presented.