23Na N.M.R. and ab initio studies of sodium cation solvation by a nitroxide radical

Abstract

There is a strong interaction between Na+ ions and the stable nitroxide radical 2,2,6,6-tetramethyl piperidinyl oxide (TMPN) in THF solution. The paramagnetic shift of the 23Na resonance indicates that positive unpaired spin density is present at the sodium nucleus. The temperature dependence of the contact shift shows that complex formation is entropy-driven, since the TMPN - Na+ complex is endothermic by 8 ± 0·4 kJ mol-1. Ab initio calculations, performed on a model system, indicate that attachment of the sodium cation to oxygen or to nitrogen leads to conformations whose energies are rather similar; only II(N) geometries are characterized by positive spin densities on sodium, as found experimentally. The longitudinal and transverse relaxation rates also point, from their relative magnitudes, to a predominant contribution from scalar I · S coupling, confirming a considerable degree of spin density transfer from the radical to the sodium nucleus.