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New, stable β‐phosphorus‐labelled pyrrolidine nitroxides from nitrones for magnetometry: an ESR investigation

The present study was devoted to the design and synthesis of new, prototypical nitroxides for use in dynamic nuclear polarization (DNP) magnetometers. Specifically, eight β‐phosphorylated pyrrolidine nitroxides were synthesized. Their large (ca. 5.0 mT) β‐31 P hyperfine splittings render them desirable for magnetometry applications. Seven of these free radicals are 2‐dialkylphosphityl‐2,5,5‐trialkyl‐ or 2‐diarylphosphityl‐2,5,5‐trialkylpyrrolidine nitroxides. Structural and solvent effects on the ESR spectra of these nitroxides were explored. Hydrolysis of a phosphityl group creates a remarkable eighth nitroxide. It exhibits an extremely pH‐dependent ESR spectrum. Other nitroxides (e.g. β‐aminyls) are known to show pH‐dependent ESR spectra (ΔaN≈0.15 mT), and these have been proposed by Keana et al. as novel, sensitive pH indicators. Whereas they operate by changes in the nitroxide nitrogen hyperfine splitting, the present special nitroxide displays little change in the nitroxide nitrogen splitting (Δa N≈0.03 mT). A very robust variation in the β‐31P hyperfine splitting (Δa N≈0.35 mT), however, is observed. Hence it may be a much more sensitive pH probe molecule than Keana et al .'s. It is proposed that this ESR variation for the present nitroxide is due to conformational changes that are pH dependent. Copyright © 1999 John Wiley & Sons, Ltd.