Quantum mechanics investigation of acid dissociation constant of carboxylic acids in aqueous solution

According to the Bronsted definition, any compound which has a hydrogen atom is an acid, since it may be lost as a proton. A thermodynamical cycle is proposed to calculate absolute pKa values for Bronsted acids in aqueous solution. The equilibrium of dissociation of a Bronsted acid depends on the interaction of the acid and its conjugate base with solvent molecules. There fore the pKa value depends on the solvent medium. The polarizable continuum model (PCM) was used to describe the solvent, and absolute pKa values were computed for different compounds: HCOOH, CH3COOH, C6H5COOH, FCH2COOH and CH3CH2COOH. The model of furnishes pKa values was in good agreement with the experimental results for some classes of compounds. The quantum Mechanics (QM) calculations were carried out with the GAUSSIAN 98 program based on HF/6-31+G** level.