Titration of nitroxide free radicals by nuclear magnetic relaxometry

Article abstract of DOI:10.1021/ac00214a027

An alternative to electron spin resonance spectroscopy is proposed for the quantitative analysis of nitroxide free radicals in solution. The method is based on the chemical reduction of the paramagnetic compounds followed by NMR measurements of the longitudinal relaxation rate of the solvent protons. This titration of the nitroxides has been carried out in ethanolic solutions by reaction with known amounts of phenylhydrazine. The paramagnetic fraction of the solvent relaxation rate is precisely related to the concentration of the free radical which can be measured without prior knowledge of its specific influence on the protons relaxation rate (relaxivity). Oxygen has to be eliminated from the solutions in order to avoid reoxidation of the hydroxylamine formed. The precision of the method, tested on 11 diversely subsituted derivatives of piperidine-1-oxyl, pyrrolidine-1-oxyl, and 3-oxazolidine-1-oxyl, offers a precision of about 3%.