Theoretical Studies On The Halogen And Chalcogen Bonding In Intermolecular Weak Interactions

Intermolecular weak interactions are stated to the intermolecular interactionsexcept for the cases of covalent, ionic and metallic bonds. Intermolecular interactionsencompass different bonding types such as hydrogen bond, C-H π interaction, π πpacking, and halogen bonding and play great roles in the fields of chemistry, materialand life science. Halogen bonding is a type of noncovalent interaction between ahalogen atom and an electronegative atom or group. Similar to halogen bonding, acovalently-bonded chalcogen atom (O, Se, Se, Te, Po) can also act as an electronacceptor to interact with an electron donor to form the chalcogen bonding. Both thehalogen bonding and chalcogen bonding have a wide range of applications in the crystalengineering, supermolecular self-assembly and life science and so on. A thoroughinvestigation on the characteristics, mechanism and bonding nature of halogen andchalcogen bonding is of great significance for designing new functional materials,designing and synthesizing new drugs and carrying on supermolecular self-assembly.In this thesis, the representative intermolecular halogen and chalcogen bondedsystems have been selected for the investigations. A systematic study on nature andcharacteristics such as geometrical features and topological properties of charge densityhas been carried out by employing several methods including “atoms in molecules(AIM)”, symmetry-adapted perturbation theory (SAPT), natural bond orbital theory(NBO) and electron localization function (ELF). The specific contents are as follows:Among the large number of charge-transfer complexes, the polar crystal adducts formed by halogen or halide compounds with molecules containing chalcogen atoms often have a significant nonlinear optical property and special intermolecular interactions. Study on the geometries, electronic properties and nature of halogen bonding in these complexes is essential for our full understanding on the properties of the complexes. In the first part, we studied the charge transfer complexes formed by iodoform (CHI3) and1,4-dioxane as well as its analogues (1,4-C2HgX2(X=O, S, Se)) by adopting quantum chemical methods with high accuracy. The results show that the interaction between I and X atoms leads to the formation of complex, and this interaction is directional, namely, along the extension of the covalent bond C-I pointing to one lone pair of X atom. AIM analysis indicates that Ⅰ… and Ⅰ… belong to closed shell noncovalent interactions, while Ⅰ…Se interaction is partly covalent in nature. NBO calculations reveal that the electron densities are transferred from the lone pairs of O (S, Se) to the σ*antibonding orbital of C-I bond. The quantities of transferred charge and the second-order perturbation stabilization energies increase in the order of O<S <Se. In these charge-transfer complexes, the induction makes the dominant contribution to the binding energy, probably because of the large amounts of transferred charge.Chalcogen bonding is a new type of noncovalent interaction. S…O and S…π interactions extensively exist in the protein structures and have a significant impact for their biochemical effects. Study on the bonding nature and geometries of S…O and S…π interactions can provide theoretical foundation for predicting protein structures and designing and synthesizing new drugs. In the second part, the complexes between FHS and O-containing compounds (H2O, H2CO and CH3OH) as well as π-electron systems (C2H2, C2H4, C4H6and C6H6) were investigated at the MP2/aug-cc-pVDZ level. All the potential chalcogen bonding structures were fully optimized and proved to be stable by vibration frequency analysis. Structurally, F-S bond stands far away from O atom or the center of π-electrons, which is considered to be attributed to the orbital interaction between the O lone pair or C-C π-electron orbital and the F-S σ*antibonding orbital. The following analysis based on ELF and NBO confirms our point of view. It is worth mentioning that on the formation of complex pairing FHS with benzene, besides this major orbital interaction mentioned above, there is also a minor one, namely, the interaction between the S lone pair and the C-C π*antibond, which leads to the electron back donation from FHS to π-electron systems. The SAPT calculations show that in S…O interaction, the induction and electrostatic terms are dominant; while in S…π interaction, the induction is the major contribution to the interaction energy, the electrostatic energy and dispersion are relatively minor and they can be comparable in strength. Notably, in the complex formed by FHS and benzene, the intervening of S-H…π attractive interaction increases the binding energy on one hand, and increases the distance between S and the center of C-C π-electrons on the other hand. The electron density topological analysis indicates that both S…O and S…π interactions are closed shell noncovalent interaction in nature.In biological systems, the strength of Se…N interaction has a dual effect on the activities of certain selenium compounds. Thus, study on the strength of Se…N interaction as well as the factors affecting such interaction is of great importance in understanding biochemical and catalytic process. In the third part, in order to discuss the effect of different X which covalently bonded to Se on the Se…N strength, complexes paired XHSe (X=CH3, NH2, CF3, CN, OCH3, OH, NO2, Cl and F) with NH3were studied. The results show that the binding energy is strikingly effected by the substituted groups and rises in the order of CH3<NH2<CF3<CN<OCH3<OH<NO2<Cl<F. The magnitudes of charge transfer and the second-order stabilization energy caused by orbital interaction depend obviously on the substituted groups as well. The induction plays a major role in stabilizing the complexes and varies intensely with the substitution. Further study finds a good linear relationship between induction and the second-order stabilization energy. The topological analysis on the electron density indicates that there is a Se…N interaction in each complex. In the complexes substituted by CH3, NH2, CF3, CN, OCH3and OH groups, Se…N interactions belong to closed shell interaction in nature; while in the complexes substituted by NO2, Cl and F, Se…N interactions are partly covalent in nature due to the large amounts of charge transfer. Furthermore, we comparatively studied Se…N interaction with the Cl (S, P)…N interactions to understand these interactions more comprehensively.