Performance of the COSMO solvation model for photoacidity and basicity in water.

The possibilities and problems to predict excited-state acidities and basicities in water with electronic structure calculations combined with a continuum solvation model are investigated for a test set of photoacids and photobases. Different error sources, like errors in the ground-state p K a $$ \mathrm{p}{K}_a $$ values, the excitation energies in solution for the neutral and (de-)protonated species, basis set effects, and contributions beyond implicit solvation are investigated and their contributions to the total error in p K a ∗ $$ \mathrm{p}{K}_a^{\ast } $$ are discussed. Density functional theory in combination with the conductor like screening model for real solvents and an empirical linear Gibbs free energy relationship are used to predict the ground-state p K a $$ \mathrm{p}{K}_a $$ values. For the test set, this approach gives more accurate p K a $$ \mathrm{p}{K}_a $$ values for the acids than for the bases. Time-dependent density-functional theory (TD-DFT) and second-order wave function methods in combination with the conductor like screening model are applied to compute excitation energies in water. Some TD-DFT functionals fail for several species to predict correctly the order of the lowest excitations. Where experimental data for absorption maxima in water is available, the implicit solvation model leads with the applied electronic structure methods in most cases for the excitation energies in water to an overestimation for the protonated and to an underestimation for the deprotonated species. The magnitude and sign of the errors depend on the hydrogen bond donating and accepting ability of the solute. We find that for aqueous solution this results generally in an underestimation in the p K a $$ \mathrm{p}{K}_a $$ changes from the ground to the excited state for photoacids and an overestimation for photobases.