Excellent response of the DFT study to the calculations of accurate relative pKa value of different benzo-substituted quinuclidines

The pKa value for protonated forms of different benzo-substituted quinuclidines in aqueous solution has been calculated using DFT methods (MP2//B3LYP, B3LYP and M06-2X) and 6-311++G(d,p) basis set and are compared with corresponding experimental values. The results were obtained from different theoretical methods so close to the corresponding experimental values but the best agreement were achieved with B3LYP/6-311+G(d,p) level of theory. The average error was found to be less than 1.6 pKa unit. Relative pKa values were computed for each protonated benzo-substituted quinuclidine using quinuclidine as a reference. Solvation was included in the calculations in all used different methods using the polarized continuum model (PCM) at the B3LYP/6-311+G(d,p) level. Furthermore, the proton affinity (PA) of neutral compounds studied here was determined. The PA values decreases as the number of phenyl groups increases.