Relationships between aqueous acidities and computed surface-electrostatic potentials and local ionization energies of substituted phenols and benzoic acids

Electrostatic potentials and average local ionization energies on the molecular surfaces of 19 phenols, 17 benzoic acids and their respective conjugate bases were computed at the HF/STO-5G(d)//B3LYP/6311G(d,p) level. Good correlations were found between pKas and the VS,max values of the neutral acids and the VS,min and IS;min of the conjugate bases for both sets of molecules. VS,max is the most positive value of the electrostatic potential on the molecular surface and is an indicator of the ease with which the phenols and benzoic acids lose their acidic hydrogens. VS,min and IS;min are the minimum values of the electrostatic potential and the local ionization energy computed on the molecular surface; the VS,min and IS;min of the conjugate bases of the phenoxides and benzoates are indicative, respectively, of the tendencies of electrophiles to approach the anions (VS,min) and to react with the anions ( IS;min) to reform the original acids. The correlations observed between the computed molecular surface quantities, taken as single parameters, and the experimental pKa values ranged from R=0.938 to R=0.970 for the two classes of compounds.