LETTERS N 2 - Radical Anion Reversibly Formed at the Surface of “Electron-Rich” Alkaline-Earth Oxides Elio Giamello* and Maria Cristina Paganini Dipartimento di Chimica IFM, UniVersita ` di Torino e Unita ` INFM di Torino, Via P. Giuria 9, 10125 Torino, Italy Mario Chiesa and Damien M. Murphy National ENDOR Laboratory, Department of Chemistry, Cardiff UniVersity, PO Box 912, Cardiff CF10 3TB, U.K. Gianfranco Pacchioni and Raffaella Soave Dipartimento di Scienza dei Materiali, UniVerista ` Milano-Bicocca, Unita ` INFM di Milano, Via R. Cozzi 53, 20125 Milano, Italy Antal Rockenbauer Chemical Research Center, Institute of Chemistry, Budapest, PO Box 17, 1525 Budapest, Hungary ReceiVed: NoVember 18, 1999 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. Introduction In the title of a recent paper by Gutsev et al., 1 the question about the existence of the N 2 - radical anion was postulated and both the stability and decay of this anion to the neutral parent molecule plus a free electron were discussed on the basis of sophisticated quantum chemistry calculations. Most of the experimental work on such radicals and other negative species are based on the interaction of target molecules with incident electrons, and the resulting short-lived species are called resonances or temporary negatiVe ions. 2 In the present Letter, © Copyright 2000 by the American Chemical Society VOLUME 104, NUMBER 9, MARCH 9, 2000 10.1021/jp994126y CCC: $19.00 © 2000 American Chemical Society Published on Web 02/09/2000