Abstract: MATH/CHEM/COMP 2002, Dubrovnik, June 24-29, 2002

Correlation of All pK Values of Polycarboxylic Acids With Atomic Charges

Ivan JuraniC¹, Zeljko Vitnik¹, Branko DrakuliC², Lidija Pfendt¹,

Gordana PopoviC³, and Branka DraZiC¹

¹Faculty of Chemistry, University of Belgrade, POB 158, 11001Belgrade, Yugoslavia

²Center for Chemistry IChTM, Njegoseva 12, Belgrade, Yugoslavia

³Faculty of Pharmacy, University of Belgrade, Yugoslavia

There are reports on the use of calculated electrostatic charges on carboxylic hydrogen for the estimation of pKa values of carboxylic acids. Various approaches could be found in the literature. Correlation has been made with charges on carboxylic hydrogen as well as with the overall charge on COOH group. Most of the mentioned reports comprise AM1 and PM3 semiempirical MO methods. The PM3 parameterization, with simulation of dielectric solvent was used in our work as a reliable method for atomic charges.

The estimation of pK values of polycarboxylic acids by the correlation with hydrogen charges was proven unsuccessful (Figure 1). Acids studied in this work were: epoxy maleic acid, epoxy fumaric acid, epoxy citraconic acid (2-methyl maleic acid), epoxy itaconic acid (2-methylene succinic acid), epoxy mesaconic acid (2-methyl fumaric acid), epoxy-cis-aconitic acid (2-carboxymethyl maleic acid) and epoxy-trans-aconitic acid (2-carboxy-methyl fumaric acid).

Correlation involves all pK values of epoxy di- and tri-carboxylic acids, i.e., including first, second and (eventual) third dissociation.

As a result of the polylinear fit the composite charge for carboxylic group is calculated as a weighted sum of atomic charges, according to the formula:

 Q = q_H + A*q_O- + B*q_C + C*q_O= + D

These charges are presented in Figure 2, showing very good linear fit. The magnitude of parameters A-D could give the insight on the mechanistic details of reactions involving carboxylic group.