Computational Studies on the Stability of [Amide]Br- Complexes

Theoretical studies on supramolecules have provided insight into their structure as well as their electronic distribution. While a number of such studies are found in the literature for neutral or cationic receptors, much less is known about anionic systems. Recently, we have published (Stibor, I.; Haffed, D. S. M.; Lhoták, P.; Hodačová, J.; Koča, J.; Čajan, M. Gazz. Chim. Ital. 1997, 127, 673) a study of amide bond activation for anion complexation where association constants were measured. In this paper, we present the results of calculations on single amides substituted on both sides of the amidic bond by an aromatic group with different substituents, as well as their complexes with anions using different methods of energy calculations. It is revealed that (i) only DFT methods give realistic results, while neither semiempirical quantum chemistry nor molecular mechanics CVFF force field give a realistic insight; (ii) the ligands (amides) themselves exhibit a different scale of nonplanarity which is caused by the repulsion between the ortho substituents of the aromatic moieties and the oxygen atom of the amidic group; (iii) the stability of the complexes is mainly correlated to three factors, the length of the hydrogen bond between the anion and amidic hydrogen, the interaction energy for the calculated complex, and the charge on the bromine atom within the complex.