The beta-phosphorus hyperfine coupling constant in nitroxides: 6. Solvent effects in non-cyclic nitroxides
Résumé
In two recent articles (Org. Biomol. Chem., 2015 and 2016), we showed that changes in the phosphorus hyperfine coupling constant a(P) at position beta in beta-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 aP of persistent non-cyclic beta-phosphorylated nitroxides decrease with the normalized polarity Reichardt's constant ENT. 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.