Many practical applications of computational chemistry involve liquid environments. Continuum-based solvation model COSMO-RS has grown in popularity as a quick and powerful tool for calculating thermodynamic properties of such liquid-phase systems. Despite the prevalence of charged molecules in many practical liquid-phase systems, few studies involving COSMO-RS have examined ionic solutes. This is in part because solvation models are thought to perform poorly for charged solutes. In this work, we show how the COSMO-RS solvation model can be used with reference data to accurately predict two important physicochemical properties involving ions in different solvents: pKas and rate constants of bimolecular nucleophilic substitution reactions. We present a method of calculating non-aqueous pKas by applying calculated solvent corrections to aqueous pKa data and fitting a systematic offset constant representing the solvation energy of the proton. Our results show that pKas in six solvents predicted by this method deviate from experimental values by a mean absolute error of less than 1 log unit. Using this technique, we also present estimates of solvation energies of the proton for solvents that lack such literature values. Finally, we show that the relative rate constants of ionic SN2 reactions can be accurately predicted using COSMO-RS calculations without the need for systematic correction parameters. These results reveal that ab initio techniques can be used in combination with experimental aqueous data to easily and accurately predict properties in non-aqueous solvents.

Calculating pKas and ionic rate constants in diverse solvents using COSMO-RS

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