Trending Acid Molecular Properties with Pka

Acid pKa encodes the proton dissociation equilibrium, an important property in various chemical processes. For example, acids can participate in reactions by donating protons to the catalyst surface in electrocatalytic reactors. Predicting the pKa of acids in different microenvironments helps advance understanding of the roles of acid proton dissociation in various chemical applications. In principle, acid proton dissociation physically relates to other molecular properties of the acid. Previous work found descriptors for predicting acid pKa such as the O-H bond length and vibrational frequency. However, the relationship between other molecular parameters and pKa has not been established.

Herein, we systematically explore the correlation between various acid molecular properties with pKa in bulk solution. We use density functional theory (DFT) modeling to determine molecular properties of 25 acids under three levels of solvation: 1) acid in the vacuum, 2) acid with explicit H₂O in vacuum, 3) acid with explicit H₂O and implicit solvation. In agreement with previous work, we found level 3 of solvation to be necessary to observe a trend between O-H bond length and vibrational frequency with acid pKa. Longer O-H bond length tends to have lower frequency, and these acids require less energy to dissociate the proton. We found no correlation between the partial charges and dipole moments of O-H with acid pKa. Future work will extend the established correlation between O-H bond length/frequency with pKa to study acid pKa at electrocatalytic interfaces.