Theoretical Investigations On The Intermolecular Interactions Among Some Compounds

In this paper, a theoretical study on nature and characteristics such as geometries, interaction energies, and electronic properties for some types of intermolecular interactions have been carried out. The main results are as follows:(1) The nature and strength of the cation-Ï€interactions between NH₄⁺ and toluene, p-cresol, or Me-indole were studied in terms of the topological properties of molecular charge density and binding energy decomposition. The results display that the diversity in the distribution pattern of bond and cage critical points reflects the profound influence of the number and nature of substituent on the electron density of the aromatic rings. On the other hand, the energy decomposition shows that dispersion and repulsive exchange forces play an important role in the organic cation (NH₄⁺)-Ï€interaction, although the electrostatic and induction forces dominate the interaction. In addition, it is intriguing that there is an excellent correlation between the electrostatic energy and ellipticity at the bond critical point of the aromaticÏ€systems, which would be helpful to further understand the electrostatic interaction in the cation-Ï€complexes.(2) Ab initio molecular orbital calculations including MP2 correlation energy has been employed to optimize the geometries of the hydrogen-bonded complexes formed by HCN (HNC) with NH₃, H₂O and HF at aug-cc-pVTZ level. It is found that there are two kinds of stable complexes bonded by intermolecular hydrogen bonds: HCN (HNC) as proton donor or as proton acceptor. The effect of basis set superposition error on the total interaction energy is smaller than 3．34kJ·mol^-1 . Natural bond orbital (NBO) analysis was performed to analyze the atomic charges and charges transfer in the monomers and complexes. The results of the decomposition of interaction energy according to SAPT2 shows that the electrostatic and induction forces dominate the intermolecular interaction, and there is a good correlation between induction forces and charges transfer.(3) Geometries, interaction energies, atomic charges and charges transfer for four hydrogen-bonded dimers between HCN and HNC (HCN-HCN, HCN-HNC, HNC-HCN and HNC-HNC) were studied at the MP2/aug-cc-pVTZ level. After MP2 electron correlation, BSSE and ZPE correction, the greatest corrected intermolecular interaction among the dimers is-25．932 kJ·mol^-1 . To reveal the nature of intermolecular interactions in the dimers, a SAPT decomposition analysis on interaction energy was performed. Based on the statistical thermodynamic method, the thermodynamic properties of the title system had been also calculated at different temperature and pressure, the results show that at an analogous environment of comets (the temperature is lower than 100．00 K and the pressure is larger than 0．01Pa), the formation process of the dimers is spontaneous, indicating that either HCN or HNC could probably exist as dimers rather than as monomers.(4) Equilibrium geometries, interaction energies, atomic charge and charge transfer for the intermolecular interactions between furan and dihalogen molecules XY(X, Y= F, Cl, Br) were studied at the MP2/aug-cc-pVDZ level. Three types of geometry are observed in these interactions: theÏ€-type geometry (â… ), in which the XY lies above the furan ring and almost perpendicularly to the C4-C5 bond of furan; theσ-type geometry (â…¡), where the X atom is pointed toward the nonbonding electron pair (n-pair) of oxygen atom in furan; theχ-type geometry (â…¢), describing a blue-shift hydrogen bond formed between the hydrogen atom of furan and dihalogen molecules XY. The calculated interaction energies show that theÏ€-type structures are more stable than the correspondingσ-type andχ-type structures. To study the nature of the intermolecular interactions, an energy decomposition analysis was carried out and the results indicate that both theÏ€-type andσ-type interactions are dominantly inductive energy in nature, while dispersion energy governs theχ-type interactions.(5) Geometries, interaction energies and electronic properties for four types of dimers (hydrogen bonded, halogen bonded,Ï€-halogen bonded, and Ï€-hydrogen bonded) between HCCF and HCCR (R=F, Cl, Br) have been studied using MP2/6-311++G (d, p) ab initio calculations. It is shown that the strength of theÏ€-hydrogen bonded dimer truns out to be greater than other types of dimers, with the interaction energies-4．611 kJ·mol^-1 for HCCF-HCCF,-4．700 kJmol^-1 for HCCF-HCCCl, and-4．850 kJmol^-1 for HCCF-HCCBr respectively at the CCSD (T)/ 6-311++G (d, p) level. In an effort to understand the nature of the intermolecular interactions prevalent in these dimers, the interaction energies were decomposed into physically distinct energy components using the symmetry adapted perturbation theory (SAPT). The dispersion force is found to be the main origin of the intermolecular interactions in typeâ… andâ…¡dimers. In the case of the typeâ…¢systems, the dispersion is the major bonding force in HCCF-HCCF and HCCF-HCCCl, while the induction energy is the most important component in HCCF-HCCBr. However, both the dispersion and the electrostatic energy play a key role in typeâ…£dimers.