Influence of the hydrogen bonding on the basicity of selected macrocyclic amines.

The optimized minimum-energy geometries of different macrocyclic amines and their protonated structures were determined by using ab initio and density functional theory (DFT) calculations. All the gas phase optimizations and energy calculations were performed at the DFT/B3LYP/6-311++G(d,p) level of theory. The HF/6-31 + G(d,p) level was used for all single point calculations in the solution phase. Geometry optimizations indicate that the most stable structures are stabilized by intramolecular hydrogen bonds. The proton affinity (PA) of macrocyclic amines is controlled by the strength of intramolecular hydrogen bonds of macrocyclic amines. These hydrogen bonds strongly influence the basicity of heteroatoms in macrocycles. The highest PA value among the studied macrocyclic amines was found to be 264.9 kcal mol⁻¹ for structure 7. This is comparable with PA of proton sponges such as 1,8-bis(dimethylamino)naphthalene. The solution phase calculations were carried out in the dimethyl sulfoxide solution as a commonly used solvent in organic reactions. Natural bond orbital analysis was performed to calculate the charge transfers and the second-order interaction energies ( E⁽²⁾) between the donor and acceptor. Quantum theory of atoms in molecules (QTAIM) was also applied to determine the nature of hydrogen bonds. QTAIM studies showed that the intramolecular hydrogen bonds in these structures are electrostatic (closed-shell) interactions as well as partially covalent and partially electrostatic in nature. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]