Electron paramagnetic resonance studies of CO –2 radicals adsorbed on MgO. Identification and structure of the species using 13C and 17O labelling

Abstract

A MgO surface containing either adsorbed O^– ions or F⁺ centres irradiated with u.v. light reacts with CO or CO₂ to form O—CO^– or CO^–₂ species respectively. Using ¹³C and ¹⁷O labelling and X and Q band modes of the spectrometer, identification of the species and characterisation of the spin density distribution in the radicals were performed. Two types of species coexist for each preparation method in different relative amounts. Their e.p.r. parameters are not very different, which suggests that O^– ions are sufficiently mobile on the surface of MgO to permit instantaneous reorientation of a (O—CO)^– product, which has oxygen in a surface vacancy and the CO group projecting from the surface, into a symmetrical CO^–₂ radical with two approximately equivalent oxygen nuclei. Their structures are only slightly different suggesting that their chemical reactivities are similar.

The sp hybridisation on the carbon atom indicates an OCO bond angle of about 125° for all species, which is slightly less than CO^–₂ trapped in formate matrices (129–130°). We suggest that the radicals are trapped at the MgO surface in a nearly symmetrical geometry with their oxygen ions stabilised by two Mg²⁺ ions and the carbon atom held away from the surface.