Prediction of the pKa values of amines using ab initio methods and free-energy perturbations

A computational study has been performed on predicting the pK(a) values for amines and alkanolamines used in CO2 absorption processes. Gas-phase energies were calculated using common basis sets at Hartree-Fock (HF), MP2, and B3LYP levels. Free energies of solvation were calculated using continuum models and Monte Carlo free-energy perturbations. Results are compared with experimental pK(a) data. While the continuum methods could reproduce trends between similar molecules, they failed to predict the overall trends for series involving amines with different numbers of amine-group hydrogens and different numbers of intramolecular hydrogen bonds. Considerably better results were obtained using free-energy perturbations. On the basis of calculations of molecular vibrations, good estimates were also achieved for changes of pK(a) with temperature.