Non-empirical calculation of the electronic energy levels of the BF3 + ion. Interpretation of the photoelectron spectrum of BF3

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.