Comparison of the solvation models COSMO and EC-RISM for the prediction of photoacidity in aqueous solution

The embedded cluster reference interaction site model (EC-RISM) and the conductor-like solvation model (COSMO) are compared in terms of their ability to predict excitation energies and photoacidity in aqueous solution. The test set includes photoacids derived from phenol, naphthol, and coumarin as well as photobases derived from quinoline. Vertical excitation energies were calculated using the approximate coupled cluster singles-and-doubles model (CC2). For the neutral and protonated forms of the photobases as well as for the neutral forms of the photoacids, EC-RISM-CC2 and COSMO-CC2 yield excitation energies that agree within approximately ±0.08 eV and are in good agreement with available experimental data. For the deprotonated forms of the photoacids, which in this study are phenolate anions throughout, COSMO significantly underestimates the effects of hydrogen bond donation in aqueous solution. In contrast, EC-RISM provides a more faithful description of these solvation effects due to its ability to model solvent distributions on an atomic level, unlike continuum approaches such as COSMO. As a result, EC-RISM performs better in predicting the photoacidity of the photoacids while yielding similar results for the photobasicity of the photobases. To approximately correct for the remaining electronic structure error of CC2, higher-order corrections were estimated from vacuum calculations with CCSDR(3) and CC3 and added to the EC-RISM-CC2 results. For the excitation energies of the photoacids, the CCSDR(3) and CC3 results exhibit significant differences, with the CC3 corrections leading to better agreement with the available experimental data in solution. However, regarding photoacidity, the estimated EC-RISM-CCSDR(3) and EC-RISM-CC3 results are very similar.