Study of proper and improper hydrogen bonding using Bader’s atoms in molecules (AIM) theory and NBO analysis

Theoretical methods have been used to study the proper and improper hydrogen bonds, considering the hydrogen bond length, interaction energy, and charge transfer, for some hydrogen-bonded complexes. Topological analysis has been performed for the complex molecules, employing Bader’s Atoms in Molecules theory. The topological properties, correlate well with the structural parameters and the interaction energies. Natural Bond Orbital analysis has been performed to study the effects of charge transfer and the behaviour of X–H bonds of the proton donor on interaction of the molecules. In the proper hydrogen bond, charge transfer takes place from lone pair electrons of the proton acceptor atom to the antibonding orbital of the X–H bond, which results in elongation. In the improper hydrogen bond, contraction of X–H bonds takes place due to the charge transfer to the remote parts of the donor molecule and as well as the competitive effect between intermolecular and intramolecular charge transfer. q 2004 Elsevier B.V. All rights reserved.