N2- Radical Anion Reversibly Formed at the Surface of “Electron-Rich” Alkaline-Earth Oxides

Physisorption of N 2 gas onto the surface of a metal oxide (MgO or CaO), containing paramagnetic trapped electron centers (F S + color centers), leads to the formation of a paramagnetic species that, on the basis of its EPR spectrum and of the related spin-Hamiltonian parameters, is identified as a N 2 - radical anion. The species in fact contains two nitrogen atoms and its g and A tensors are in agreement with what observed for the N 2 - radical trapped in irradiated crystal of various azides. The surface N 2 - species is formed by surface-to-molecule one-electron transfer, and its stability strictly parallels the stability of the physisorbed layer, the species formation being completely reversible and pressure dependent. When the N 2 adlayer is desorbed, in fact, the N 2 - spectrum vanishes and the original F s + spectrum is restored. Ab initio quantum chemical calculations on an embedded MgO cluster fully confirm the observed phenomenon indicating, in agreement with EPR analysis, the electron transfer of a large fraction of electron density into the π orbitals of the admolecule.