Protonation of group 14 oxides: The proton affinity of SnO

Abstract

Molecular orbital calculations are reported for the monoxides, XO, of group 14 elements ( X = C, Si, Ge and Sn) and for both isomers, XOH + and HXO +, of the protonated monoxides. Structure optimisation has been carried out using the Density Functional Theory employing the B3LYP procedure and at both Hartree-Fock and MP2 (full) levels, all with a variety of medium-sized Gaussian basis sets. In all XO molecules the oxygen atom is the preferred site for protonation, except when X = C where HCO + is the lower energy isomer. Barriers to interconversion between the two isomers XOH + and HXO + are overestimated by the Hartree-Fock calculations, but with wave functions that include electron correlations they generally fall into the range 27–44 kcal mol −1. Proton affinities increase as the atomic number of X increases, and values calculated by averaging over all wave functions that include electron correlation, 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).