Hyperfine Coupling on Nitrogen and Phosphorus in β-Phosphorylated Nitroxide Radicals: Calculations and Models

This work presents computational experiments rationalizing structure–property correlations encountered in the rich experimental data on electron paramagnetic resonance (EPR) of β-phosphorylated nitroxide radicals, looking particularly at the hyperfine coupling parameters (HFC), proportional to the spin density at N and P nuclei. The models are based on the complementary use of density functional theory (DFT) and valence bond (VB) calculations. Taking a sufficiently large space of resonance structures, the VB account of HFCs becomes quantitatively comparable with the unrestricted DFT results, in a range compatible with experiment. At nitrogen, the HFC includes spin-polarization effects and a part tuned by the s:p content of the spin carrier, determined by the local molecular geometry embedding the NO group. The analysis performed on models of the title compounds showed that the spin density arrives on the phosphorus nucleus, in traces, by through-space distant interactions with the nitrogen spin carrier. The geometry dependence of the HFC parameters was charted on models relevant for a large class of instrumentally characterized molecules.