PROTONATION OF GROUP-14 OXIDES - THE PROTON AFFINITY OF SNO

Molecular orbital calculations are reported for the monoxides, XO, of group 14 elements (X = C, Si, Ge and Sn) and for both isomers, XOH+ an d HXO+, of the protonated monoxides. Structure optimisation has been c arried out using the Density Functional Theory employing the B3LYP pro cedure and at both Hartree-Fock and MP2 (full) levels, all with a vari ety of medium-sized Gaussian basis sets. In all XO molecules the oxyge n atom is the preferred site for protonation, except when X = C where HCO+ is the lower energy isomer. Barriers to interconversion between t he two isomers XOH+ and HXO+ are overestimated by the Hartree-Fock cal culations, but with wave functions that include electron correlations they generally fall into the range 27-44 kcal mol(-1). Proton affiniti es increase as the atomic number of X increases, and values calculated by averaging over all wave functions that include electron correlatio n, give the following proton affinities: for CO, 141.5; for SiO, 189.3 ; for GeO, 196.1; and for SnO, 215.6 (all in kcal mol(-1)). (C) 1997 E lsevier Science B.V.