β-Phosphorus Hyperfine Coupling Constant in Nitroxides: Conformational Effects in 6-Membered Ring Nitroxides

Recently, we reported a significant solvent effect on the phosphorus hyperfine coupling constant a P in β-phosphorylated 6-membered ring nitroxides (∆a P = 24 G in Org. Biomol. Chem. 2015). Thus, it led us to investigate the effect of solvent for several 6-membered ring nitroxides. Although smaller than mentioned above, a change of 5–7 G in a P with the polarity of solvent was still observed for these nitroxides. As for other β-phosphorylated nitroxides, a N increases with the polarity/polarizability π* and the Hydrogen Bond Donating α properties of the solvent whereas a P exhibits the reverse trends. The change of a P with the solvent depends a lot on a subtle interplay between the destabilizing steric hindrance due to the bulkiness of the substituents and the stabilizing hyperconjugation interactions SOMO → σ*C–P between the anti-bonding orbitals of the C–P bond and the SOMO.