THEORETICAL-STUDY OF METHANE ACTIVATION BY GAS-PHASE CATIONIC MAIN-GROUP METAL-OXIDE DIATOMICS

With the us+e of semi-empirical MNDO and PM3 molecular orbital calcula tions, activation energies and transition state geometries have been f ound for C-H bond activation in CH4 by the ''superbasic'' oxometal cat ionic species LiO+, BeO+, MgO+ and AlO+ in the gas phase. The most imp ortant parts of the potential energy hypersurfaces along with the intr insic reaction paths and the energetics of the reactions, have been ca lculated for three different reaction geometries corresponding to both the collinear O and M end-on and the M-O side-on approaches of the C- H bond. Depending on the nature of the oxometal cationic species, the final products and/or intermediates involved along the [M, O, C, H-4] potential hypersurface were found to be either MOH+ (g) + CH3 . (g), the insertion product CH3MOH+ (g), or methanol coordinated to the meta l ion, M(CH3OH)+ (g). All reactions follow the currently accepted reac tion profile for a gas-phase ion-molecule reaction in which the transi tion states are preceded by ''loose'' ion-molecule complexes. In all c ases the activation barriers were calculated to be lower for the colli near than the parallel reaction geometry and follow the trend: AlO+ > BeO+ > MgO+ > LiO+. These results were rationalized on the basis of st ructural and electronic features of the transition states with the lat ter involving different blends of electrostatic and covalent interacti ons. Along these lines the most significant point that emerged was tha t some form of catalysis, a hole or open-shell catalysis, is essential in order to effect C-H bond activation in methane.