Modeling Temperature Dependency of Ionization Constants of Amino Acids and Carboxylic Acids

The temperature sensitivity of pKa values for a data set of 10 amino acids and 5 carboxylic acids is studied using PCM and SM8T continuum solvation models coupled with density functional theoretical calculations at the B3LYP/6-311++G(d,p) level of theory. The data set of amino acids was chosen on the basis of their potential to be solvents for postcombustion CO₂ capture processes and available literature data. Calculated results are compared with experimental data in a temperature range of 273–393 K. Both solvation models predict temperature sensitivity of pKa of the amino group of amino acids very nicely. It is observed that the temperature dependencies of pKa of the carboxyl group of amino acids and carboxylic acids predicted by these models do not agree well with experimental temperature dependencies of carboxylic acids. This issue is discussed in the context of the basic parametrization of these continuum solvation models.