Accurate relative pK_a calculations for carboxylic acids using complete basis set and Gaussian-n models combined with continuum solvation methods

摘要

The complete basis set methods CBS-4, CBS-QB_3,and CBS-APNO, and the Gaussian methods G2 and G_3 Were used to calculate the gas phase energy differences between six different carboxylic acids and their respective anions. Two different continuum methods, SM_5.42R and CPCM, were used to calculate the free energy differences of sol,,:ation for the acids and their anions. Relative pK1 values were calculated for each acid using one of the acids as a reference point. The CBS-QB_3 and CBS-APNO gas phase calculations, combined with theCPCM/HF/6-31+G(d)//HF/6-3IG(d) 01 CPCM/HF/6-31+G(d)//HF/6-31 +G(d) continuum solvation calculations ollthe lowest energy ga~ phase conformer, and with the conformationally averaged values, give results accurate to 1/2pK_a unit.