Solvent Effect in β-Phosphorylated Nitroxides: Model Nitroxides

Solvent effects have been investigated for the last four decades. As far as we know, only 3 articles have reported on the solvent effect with β-phosphorylated nitroxides. In 1976, Il’Yasov and coll. (Theor Exp Chem 12:656, 1976) showed in 16 solvents that the phosphorus hyperfine coupling constant aP linearly decreased with the polarity of the solvent given by the normalized Reichardt–Dimroth polarity constant ENT for in situ generated nitroxides, e.g., from aP = 40.80 G in n-hexane (ETN = 0.09) to aP = 22.80 G in water (ETN = 1). A few years later, Janzen and coll. (Magn Reson Chem 37:251–258, 1999) reported a weak (around 4 G) decrease in aP for a few β-phosphorylated nitroxides in n-hexane and methanol. Recently, our group reported hardly any change in aP with the solvent for a very stable nitroxide, e.g., aP = 46.19 in n-pentane and aP = 45.60 in water. In this article, we report the solvent effect in 40 solvents on two new nitroxides and discuss at the light of the results reported in the literature. It turns out that aP changes obey no trends, i.e., strong or weak decrease, no change or even a weak increase in aP was observed depending on the structure of each nitroxide.