Artificial Photosynthesis

Toward the prediction of accurate pKa values in water and acetonitrile

We have developed a simple approach for the calculation of accurate pK_a values in water and acetonitrile based on the straightforward calculation of the gas-phase absolute free energies of the acid and conjugate base with use of only a continuum solvation model to obtain the corresponding solution-phase free energies. Most of the error in such an approach arises from inaccurate differential solvation free energies of the acid and conjugate base which is removed in our approach using a correction based on the realization that the gas-phase acidities have only a small systematic error relative to the dominant systematic error in the differential solvation. The methodology is shown to generally predict the pK_a values for all the cases investigated to within 1 pH unit so long as the differential solvation error is larger than the systematic error in the gas-phase acidity calculations. Such an approach is rather general and does not have additional complications that would arise in a cluster-continuum method, thus giving it strength as a simple high-throughput means to calculate absolute pK_a values.

Biochim. Biophys. Acta - Bioenergetics 2013, 1827, 882-891, DOI:10.1016/j.bbabio.2012.11.004.