Gas phase acidities and molecular geometries of H3SiOH, H3COH, and H2O

Abstract

Theoretical predictions are made by the coupled pair functional method employing large basis sets, [6s,5p,2d,1f/5s,3p,2d,1f/3s,2p] for geometries and [6s,5p,2d,1f/8s,5p,3d,1f/4s,2p] for deprotonation energies. The geometries obtained for H2O and CH3OH agree with the observed ones within the experimental uncertainty limits. The predicted equilibrium geometry (picometers, degrees) for silanol is r(SiO)=165.0, r(OH)=95.76, r(SiH)=146.8 (in plane), r(SiH)=147.6 (out of plane), angle (SiOH)=117.7. Nuclear motion corrections to geometries and vibrational contributions to deprotonation energies are considered. The predicted gas phase acidities (ΔH0298) are 392.3 (H2O), 384.5 (CH3OH), and 359.3 (SiH3OH) kcal/mol. The differences are discussed in terms of the potential on the acidic proton as well as of contributions from rearrangement of electrons and relaxation of nuclei. The remaining basis set error is assessed on calculations for H2O which employ a [10s,8p,4d,2f,1g/7s,3p,1d] basis set. The best theoretical estimate of ΔH0298 for H2O is 391.0±2.5 kcal/mol.