COMPARISON OF ABSOLUTE ACIDITIES AND BASICITIES OF METHANOL AND SILANOL CALCULATED BY THE AB INITIO MO METHOD

Abstract

Absolute gas-phase basicities and acidities were calculated at the RHF/3-21G/ /3-21G, RHF /3-21+G/ /3-21G, RHF /3-21+G/ /3-21+G and RHF /3-21+G(*)/ /3-21G(*) levels of ab initio theory. The effect of the use of diffuse and polarization functions on the calculated geometries, acidities and basicities was studied. The use of diffuse functions is necessary to calculate accurate acidity and it has a 10 kcal/mol effect on the calculated basicity. The use of d-functions (3-21 G(*) basis set) for silanol has an important effect on the calculated acidity and basicity. The d-functions play a lesser role if the polarization functions are used. The absolute gas-phase acidity and basicity of silanol have not been measured until now; we give a prediction for those values within 3 kcal/mol. According to the calculations, silanol is more acidic than methanol, the difference being 22 kcal/mol. The basicity of the two molecules is closer to each other, the protonation energy of silanol being 6 kcal/mol less. According to the 3-21 G(*) calculations the Si-O bond length increase is very important during protonation of silanol.