A theoretical study on the pKa values of selenium compounds in aqueous solution

Abstract

The importance of selenium compounds in chemistry and biochemistry has become apparent. The pK_a values are important for understanding the reactivity and applicability of selenium compounds. To this end, we calculated the the pK_a values of 16 selenols in aqueous solution by using two kinds of DFT methods and PCM-Bondi, SMD and SMD_sSAS models. It is observed that the highest precision with a lowest RMSE value of 0.38 pK units was provided by using the ωB97XD method with the SMD model. Therefore, this theoretical protocol was employed to calculate the pK_a values of selenol compounds and selenoxo forms of selenocarboxylic acid compounds including R = alkyl, cycloalkyl, benzyl, allyl and aryl as well as the substituent effects. The results suggest that selenols can give higher pK_a values than selenocarboxylic acids. Generally, for both substituted selenols and selenocarboxylic acids, EWGs can decrease the pK_a values and EDGs can increase the pK_a values, and the pK_a of the EWG of –COOH is larger and the pK_a values of the EDGs of –NH₂ and –N(CH₃)₂ are smaller. In addition, linear relationships between the pK_a values with the natural charges of Se atoms, E_HOMO, etc. were obtained in different types of selenols and selenocarboxylic acids.