Improper Hydrogen Bonds Research Articles

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.

First local minimum of the formic acid dimer exhibits simultaneously red-shifted O–H⋯O and improper blue-shifted C–H⋯O hydrogen bonds

The first local minimum of the formic acid dimer exhibits simultaneously red-shifted O–H⋯O and blue-shifted C–H⋯O hydrogen bonds. The improper, blue-shifted hydrogen bond was investigated by means of the NBO analysis, rehybridization model and optimization in the electric field. It was shown that the electrostatic model cannot describe the nature of the blue-shifted H-bond. From the NBO analysis it becomes evident that the formation of the O–H⋯O hydrogen bond and C–H⋯O improper hydrogen bond can be explained on the basis of an increase of electron density in the σ* antibonding O–H orbital and a decrease of electron density in the σ* antibonding C–H orbital. While the former effect is easily explained on the basis of hyperconjugation, the latter requires the existence of a new mesomeric structure characterized by delocalization of electron density from the C–H σ* antibonding orbital to the remaining part of the complex. The rehybridization model explains properly the formation of both hydrogen bonds but fails to interpret the changes of the other bonds.

C–H⋯X interactions of fluoroform with ammonia, water, hydrogen cyanide, and hydrogen fluoride: conventional and improper hydrogen bonds

The structural changes, dipole moments, and interaction energies for a series of C–H⋯X hydrogen bonded complexes involving F3CH as proton donor and NH3, OH2, NCH, and FH as proton acceptors have been studied using ab initio calculations including electron correlation effect for various basis sets. The NH3 shows a conventional hydrogen bond, while the other molecules show improper hydrogen bonds as evidenced by the C–H bond length shortening and the blue-shift of C–H stretching vibrational frequency. In the complex of F3CH⋯NCH, both C–H and N–C bonds are contracted, and their corresponding stretching vibrational frequencies are blue-shifted compared with monomers. The blue-shifted stretching vibration in the proton acceptor molecule has not been much addressed until now. Both conventional and improper hydrogen bonds are noted in the nonlinear complex of F3CH⋯FH, where FH molecule plays a dual role of proton acceptor and donor at the same time. The interaction energies are almost equivalent for HF and MP2 calculations, which does not support a recent proposition that the dispersive interaction is the origin of improper H-bonding.

Hydrogen bonds, improper hydrogen bonds and dihydrogen bonds

The structures, interaction energies and vibrational spectra of a large number of molecular complexes, formed by binary combination of the covalent hydrides of some of the elements of the first two rows of the periodic table, have been determined by means of ab initio molecular orbital theory at the MP2 level, using the 6-311++G(d,p) basis set. The results are discussed in terms of a variety of different types of interaction experienced by the monomer species as they undergo association, namely conventional hydrogen bonding, improper hydrogen bonding, dihydrogen bonding and electron donor–acceptor interaction.