Hyperfine coupling constants of β-phosphorylated nitroxides: a tool to probe the cybotactic effect by electron paramagnetic resonance

Nitrogen hyperfine coupling constants (hcc) a(N) of nitroxides measured by electron paramagnetic resonance are used to probe the cybotactic effect of solvents. A few articles (Janzen et al., Can. J. Chem. in 1982 for a(Hβ) and Deng et al., J. Fluorine Chem. in 2002, both for a(Hβ) and a(Fβ)) dealt with the cybotactic effect on the hcc of atoms (hydrogen H, fluorine F, and phosphorus P) in β positions of nitroxides. They show either no significant change or an increase in a(β) with a(N) . However, an early report by Il'Yasov et al. (Theor. Exp. Chem. 1976, 656) described a dramatic change in a(Pβ) with the solvent and an anti-correlation of a(Pβ) with a(N) . We decided to reinvestigate that work using the N-(2-methylpropyl)-N-(1-diethylphosphono-2,2-dimethylpropyl)-N-oxyl radical that carries both hydrogen and phosphorus atoms on the same carbon attached to the nitroxy moiety and that is highly persistent, in contrast to the nitroxide used by Il'Yasov. We observe the same trends as those observed by Il'Yasov, except that the solvent cybotactic effect on a(N) and a(Pβ) is dramatically weaker in our case. We showed that a(Pβ) is correlated to both the cohesive pressure c and the normalized Reichardt polar constant E(T)(N), and that hydrogen-bonding donor solvents exhibit a specific behavior.