Continuous wave electron paramagnetic resonance investigation of the hyperfine structure of 17O2− adsorbed on the MgO surface

The adsorption of molecular oxygen ~enriched with 17O) onto high surface area MgO has been\ud studied by electron paramagnetic resonance ~EPR! spectroscopy. The oxide surface was pretreated\ud in such a way so that surface trapped electron FS\ud 1~H! centers are produced. Subsequent dioxygen\ud adsorption results in an electron transfer reaction from FS\ud 1~H! centers to O2, producing a surface\ud stabilized superoxide (O2\ud 2) anion. The resulting EPR spectrum of the paramagnetic anion is\ud complicated by the simultaneous presence of a high number of ‘‘normal’’ hyperfine lines along the\ud principal axes and also by several off-axis extra features which have complicated previous\ud interpretations of the Ayy and Azz components. By adopting a suitable adsorption procedure which\ud suppresses the superoxide speciation, using a highly crystalline MgO material and controlling the\ud isotopomer composition through appropriate 17O enrichments, the resolution of the EPR spectrum\ud has been dramatically improved. Analysis of the 1H superhyperfine structure (uAHu/beg\ud 5@3.9,2.2,1.3#G), resulting from a dipolar interaction between the adsorbed O2\ud 2 anion and a\ud neighboring OH group, and positions of the extra absorption lines in the spectrum, have provided us\ud with auxiliary sources of information to determine for the first time the complete 17O hyperfine\ud tensor (AO/beg5@276.36,7.18,8.24# G!. The tensor has been analyzed in detail using a localized\ud spin model. The spin density is shared among the 2p p\ud x (0.495), 2px\ud y(20.024) and 2s(0.011)\ud orbitals. The total spin density on O2\ud 2 indicates that a complete surface electron transfer from the\ud FS\ud 1~H! center to dioxygen occurs upon adsorption, in line with recent ab initio calculations.