The β-phosphorus hyperfine coupling constant in nitroxides: 6. Solvent effects in non-cyclic nitroxides

Abstract

In two recent articles (Org. Biomol. Chem., 2015 and 2016), we showed that changes in the phosphorus hyperfine coupling constant a_P at position β in β-phosphorylated nitroxides can be dramatic. Such changes were applied to the titration of water in organic solvents and conversely of organic solvents in water. One of the molecules tested was a non-cyclic nitroxide meaning that a thorough investigation of the solvent effect on the EPR hyperfine coupling constant is timely due. In this article, we show that the a_P of persistent non-cyclic β-phosphorylated nitroxides decrease with the normalized polarity Reichardt's constant E^N_T. The Koppel–Palm and Kalmet–Abboud–Taft relationships were applied to gain deeper insight into the effects influencing a_N and a_P: polarity/polarizability, hydrogen bond donor properties, and the structuredness of the cybotactic region.