Theoretical studies on the isomers and tautomers of 22-membered macrocyclic ligand

Density functional theory (DFT) calculations at B3LYP/6-31+G(d) in the gas phase and ethanol have been used to study different isomers and tautomers of a novel macrocyclic ligand (L) synthesized recently from diethyloxalate and dipropylenetriamine. The relative stability of 21 isomers and tautomers shows that this compound exists mainly, in the gas phase and ethanol solution, as a tetra-keto tautomer. The most stable form has all CO and NH moieties in anti configurations with each other. Flipping of one NH group to the syn position destabilizes the resulting isomer by 11 kcal/mol with an energy barrier of 38 kcal/mol. Enolization processes are hindered by higher energy barriers of 42 and 45 kcal/mol in the gas phase and ethanol, respectively. Charges on different atoms and NBO analysis, were used to explain the electronic structures of these systems. The calculated structures and frequencies of the ligand agree with the previously reported experimental data.