Theoretical Studies Of Intermolecular Interactions: Hydrogen Bond, Van Der Waals Contact And Halogen Bond

Intermolecular interactions are very active research areas which play important roles in chemistry, physics and biology, and so on. And many experimentalists and theorists have paid their attention to these fields. In the thesis, three classes of intermolecular interactions, namely, hydrogen bond, van der Waals interaction and halogen bond have been selected for the investigations. With respect to the hot topics related to these three intermolecular interactions, different theoretical methods have been adopted to study the properties and natures of them The main results are as follows:1. The cis-trans noncyclic formic acid dimer was studied by means of MP2 method with 6-31G(d,p),6-31+G(d,p) and 6-311+G(d,p) basis sets. It exhibits simultaneously red-shifted O-H…O and blue-shifted C-H…O hydrogen bonds. AIM and NBO analyses were performed at the MP2/6-31+G(d,p) level to explore their properties and origins. AIM analysis provides the evidence that the O-H bond becomes weaker and the C-H bond becomes stronger upon the hydrogen bond formations. Intermolecular and intramolecular hyperconjugations have important influence on the electron densities in the X-H (X=O, C)σbonding orbital and itsσ* antibonding orbital. The electron densities in the two orbitals are closely connected with the X-H (X=O, C) bond length, and they have been used to quantitatively estimate the bond length variation. The larger amount of charge transfer in the red-shifted O-H…O hydrogen bond is due to its favorable H…O electron channel, whereas the H…O electron channel in the blue-shifted C-H…O hydrogen bond is weaker. Structural reorganization effect shortens the C-H bond by approximately 30% when compared to the C-H bond contraction upon the dimerization. Strikingly, it leads to a small elongation and a slight red shift of the O-H bond. Both rehybridization and repolarization result in the X-H (X=O, C) bond contraction, but their effects on the O-H bond do not hold a dominant position. The hydrogen-bonding processes go through the electrostatic attractions, van der Waals interactions, charge-transfer interactions, hydrogen-bonding interactions and electrostatic repulsions. Electrostatic attractions are of great importance on the origin of the red-shifted O-H…O hydrogen bond, especially the strong Hδ+…Oδ- attraction. For the blue-shifted C-H…O hydrogen bond, the considerable nucleus-nucleus repulsion between H and O atoms caused by the strong electrostatic attraction between C and O atoms is a possible reason for the C-H bond contraction and its blue shift.2. Standard Watson-Crick adenine-thymine (AT) base pair has been investigated by using the B3LYP functional with 6-31G(d, p) basis set, at which level of theory the geometrical characteristics of AT base pair are the best in agreement with the experiment. It exhibits simultaneously red-shifted N-H…O and N-H…N hydrogen bonds as well as a blue-shifted C-H…O contact. AIM analysis suggests that the blue-shifted C-H…O contact exists as van der Waals interaction, and the electron densityρthat reflects the strength of a bond has been used to explain the red- and blue-shifted. By means of NBO analysis, we report a method to estimate the effect of hyperconjugation quantitatively, which combines the electron density in the X-H (X=N, C)σbonding orbital with that in theσ* antibonding orbital. The effect of structural reorganization on the origins of the red- and blue-shifted has been considered by the partial optimization, its behavior on the X-H (X=N, C) bond is quite different. Rehybridization and repolarization models are employed and they act as bond-shortening effects. The competition between the electrostatic attractions and Pauli/nucleus repulsions is present in the two typical red-shifted N-H…O and N-H…N hydrogen bonds as well as in the blue-shifted C-H…O van der Waals contact. Electrostatic attraction between H and Y atoms (Y=O, N) is an important reason for the red shift, while the nucleus-nucleus repulsion between H and O atoms may be a factor leading to the C-H bond contraction and its blue shift. The electric field effect induced by the acceptor O atom on the C-H bond is also discussed.3. A series of ringy complexes formed by H3N, HF and XY (X=C1, Br and Y=F, Cl, Br) have been studied by means of ab initio MP2 method with aug-cc-pVTZ basis set. The geometries, stretching vibrational frequencies and the interaction energies of these complexes are investigated. It shows that each complex has two red-shifted hydrogen bonds and one red-shifted halogen bond. AIM analysis has been performed to examine the topological characteristics at the bond critical point (BCP) as well as those at the ring critical point (RCP). It confirms the coexisting two hydrogen bonds and one halogen bond in each complex. The effects of hyperconjugation, rehybridization and repolarization on the bonds involved in the hydrogen bond and halogen bond are discussed in detail. Molecular electrostatic potential (MEP) is used to explore the mechanisms of the formations of the hydrogen bonds and halogen bonds and the ringy structures.