Semi-empirical studies of substituent effects on the ionization of bicyclooctane carboxylic acids and quinuclidines

Semiempirical AM1 calculations were performed for a representative series of 4-substituted bicyclooctane carboxylic acids and quinuclidines. It was found that the Hammett constant, σ_I, and the Swain and Lupton field constant, F, correlate linearly with the differences in the heat of formation of isodesmic reactions. These constants also correlate with the charges on the acid moiety of the bicyclooctane acids and their anions, and with the hydrogen net charge on the protonated quinuclidines. For all cases, the NO₂ was the poorest correlated substituent.