Quantum chemical estimation of the acidities of some inorganic oxoacids

Abstract

Oxoacids are compounds in which the acid dissociation process occurs through rupture of an oxygen–hydrogen bond. Although many computational studies have examined the acidities of organic oxoacids, relatively few have been directed at inorganic oxoacids. In this work, an attempt is made to find suitable molecular descriptors that correlate with the acidities (as represented by pK_a values) of a diverse set of inorganic oxoacids and can thus be employed to estimate their acidities. Calculations were performed at the DFT B3LYP/6-31+G^** level, with and without the use of the SM8 aqueous solvent model. Atomic charge descriptors, which have performed well in past studies, did not correlate well with the present experimental acidities. However, the energy difference ΔE(H₂O) between the neutral oxoacid and its corresponding anion, determined within the SM8 aqueous solvent model, provided strong correlations (R² = 0.987) with the experimental pK_a values. It appears that ΔE(H₂O) acts as a reasonable surrogate for variations in the Gibbs energy change ΔG(H₂O) of the acid dissociation process HA A⁻ + H⁺ for the majority of the inorganic oxoacids considered in this study. At the same time, significant deviations from this correlation may indicate the participation of processes other than the standard Brønsted acid dissociation mechanism.

Keywords:

• oxoacid
• inorganic
• acidity
• pK_a
• computation

Acknowledgements

The author thanks Dr Jane S. Murray (University of New Orleans) and Prof. Catherine Housecroft (University of Basel) for their helpful correspondences and comments on this topic.