Hyperfine Coupling Constants of beta-Phosphorylated Nitroxides: A Tool to Probe the Cybotactic Effect by Electron Paramagnetic Resonance
Résumé
Nitrogen hyperfine coupling constants (hcc) aN 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 aH beta and Deng et al., J. Fluorine Chem. in 2002, both for aH beta and aF beta) dealt with the cybotactic effect on the hcc of atoms (hydrogen H, fluorine F, and phosphorus P) in beta positions of nitroxides. They show either no significant change or an increase in a beta with aN. However, an early report by Il'Yasov et al. (Theor. Exp. Chem. 1976, 656) described a dramatic change in aP beta with the solvent and an anti-correlation of aP beta with aN. 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 aN and aP beta is dramatically weaker in our case. We showed that aP beta is correlated to both the cohesive pressure c and the normalized Reichardt polar constant ETN, and that hydrogen-bonding donor solvents exhibit a specific behavior.