For decades, the nitrogene hyperfine coupling constant aN of nitroxides has been applied to probe their environment using EPR. However, the small changes observed (≈2 G from n-pentane to water) with the solvent polarity allow only a qualitative discussion. A stable β-phosphorylated nitroxide exhibiting a small change in aN (≈3 G from n-pentane to water) and a striking change (≈25 G from n-pentane to water) in phosphorus hyperfine coupling constant aP with the polarity of solvent was prepared and used to develop the first procedure for the titration of water in THF by EPR, down to 0.1% v/v.
