Computational studies on the stability of [amide]Br- complexes

Theoretical studies on supramolecules have provided insight into their stru cture as well as their electronic distribution. While a number of such stud ies are found in the literature for neutral or cationic receptors, much les s is known about anionic systems. Recently, we have published (Stibor, I.; Haffed, D. S. M.; Lhotak, P.; Hodacova, J.; Koca, J.; Cajan, M. Gazz. Chim. Ital. 1997, 127, 673) a study of amide bond activation for anion complexat ion where association constants were measured. In this paper, we present th e results of calculations on single amides substituted on both sides of the amidic bond by an aromatic group with different substituents, as well as t heir complexes with anions using different methods of energy calculations. It is revealed that (i) only DFT methods give realistic results, while neit her semiempirical quantum chemistry nor molecular mechanics CVFF force fiel d give a realistic insight; (ii) the ligands (amides) themselves exhibit a different scale of nonplanarity which is caused by the repulsion between th e ortho substituents of the aromatic moieties and the oxygen atom of the am idic group; (iii) the stability of the complexes is mainly correlated to th ree factors, the length of the hydrogen bond between the anion and amidic h ydrogen, the interaction energy for the calculated complex, and the charge on the bromine atom within the complex.