The Study of ab initio theory and PCM–UAHF model for Prediction of the pKa for Silanoic acids derivatives in aqueous solution

The acid–base properties of organic molecules affect their toxicity [1], chromatographic retention behavior [2] and pharmaceutical properties. In addition the acidity constants play a very fundamental role in many analytical procedures such as acid base titration, solvent extraction, complex formation and ion transport. Therefore, having the knowledge about acid–base properties of chemical substances gives an idea about their toxicity, pharmaceutical activity, and analytical roles. Much of the theoretical foundation of modern physical organic chemistry is based on the observation of the effects on acid–base equilibrium of changing molecular structure. Proton transfer reactions constitute an important class of chemical reactions and have been studied extensively over a long period of time [3]. These studies in a gas phase and solution phase provides information on inherent substitute effects and solute–solvent interactions. In these work Ab initio molecular orbital methods at the CBSEQ level of theory have been used to study the structure and acidity of silanoic acids derivatives in aqueous solution. The calculations are performed at the SCF level with inclusion of SCF–level thermochemical corrections to yield free energies of dissociation, using the basis sets 6-31G** and the recently parameterized continuum–solvation method PCM–UAHF for the solvation contribution. The model furnishes pKa values in relatively good agreement with experimental data. Scaling different parts of solvation energies provides a significant improvement in results and signifies the importance of balance of individual contributions from electrostatic, cavity, dispersion and repulsion interactions. The result show that the investigated acids are weak and for Si-acids the acidity increases inthe order: CCl3SiOOH > CHCl2SiOOH > FSiOOH > ClSiOOH > HSiOOH > BrSiOOH > CH2ClSiOOH > CH3SiOOH > NO2SiOOH > NH2SiOOH