THEORETICAL-STUDIES OF PKAS FOR ORGANIC-COMPOUNDS IN AQUEOUS-SOLUTION

pKa differences between formic acid and acetic acid and between methanol and methanethiol were calculated as a sum of the acidity difference in the gas phase and the solvation free-energy difference. The ab initio molecular-orbital method with MP3/6-31+G* and/or MP4/6-31+ +G** basis sets and the molecular-dynamics free-energy perturbation technique were applied to the studies of the acidity and solvation, respectively. The estimated value for formic acid and acetic acid was in good agreement with the experimental value within a few pKa units, but that for methanol and methanethiol was different from the experimental value by 4-6 pKa units. © 1990.