Preferred conformations of the gas phase complex between Li+ and a model macrocycle tetraamide

The gas-phase binding of Li+ by a model macrocycle tetraamide, via its amide CO groups, is investigated using ab initio methods. The macrocycle ligand contains four amide units linked by alternating phenyl and ethenyl groups. Geometries and vibrational frequencies for the complexes are obtained at the B3LYP/6-31G(d) level. Geometries are further optimized at the B3LYP/6-31+G(d) level. Interaction energies, corrected for basis set superposition error, are evaluated using the MP2 method with the 6-31+G(d) and 6-311+G(d) basis sets. It is found that the model tetraamide presents enough flexibility to bind the Li+ metal ion in at least four different conformations characterized by their group symmetries Cs, C2, C2h, and C2v. The respective interaction energies are close to one another with an average value of −79.40 kcal/mol at the MP2/6-311+G(d) level. Natural bond orbital analysis (NBO) is performed to investigate the extent of ligand polarization and charge-transfer (CT) interactions in the various conformations of the complex. Extended conformational searches for the free macrocycle tetraamide, and its Li+ complex conformations are conducted using a Monte Carlo-based analysis with the PM3 semiempirical method and compared with the B3LYP results via similarity calculations.