The novel exo-methylene pyrrolidinyl nitroxide 7, meanwhile, exhibits slightly faster reduction kinetics than the benchmark pyrrolinyl nitroxide 5 (6% remaining radical). This suggests that any difference in inherent stability of the exo-methylene pyrrolidinyl nitroxide scaffold compared to the pyrrolinyl scaffold can be compensated by a conformational shift of the opposite effect. Compared to ester 7, however, the carboxylic acid 8 exhibits the greatest stability among the examined spirocyclic nitroxides remaining radical). Under the experimental conditions this compound is present in the anionic carboxylate form, is known to provide stabilization through inductive electronic and electrostatic effects. These effects clearly have a much larger impact on reduction rates than the saturation and substitution pattern of the nitroxide core ring.