STRUCTURE-ACIDITY RELATIONSHIPS IN 3-SUBSTITUTED AND 4-SUBSTITUTED BENZOIC-ACIDS STUDIED BY PM3-CALCULATIONS

In this paper the semi-empirical PM3 computation scheme is used to inv estigate the relationship between the electronic and molecular structu re and the acidity of 3- and 4-substituted benzoic acids. The acidity is well reproduced by the semi-empirical enthalpy of the dissociation. Moreover, the substituent effect on the acidity of benzoic acids is c losely mirrored by the small changes induced in the electronic and mol ecular structure of the carboxy moiety. Several computed parameters in volving the carboxy moiety, such as frontier molecular orbital energy, atomic or group charge, bond length and angle, are linearly correlate d to the acidity. Some of these parameters can be reliably used to pre dict the acidity of unknown systems. At variance with previous results , we found that the substituent influences to the same extent the neut ral and anionic forms of the investigated systems. In conclusion, semi -empirical methods are still worthwhile in the elucidation of chemical problems.