Study On The Nature Of Some Intermolecular Interactions

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 of the hydrogen-bonded and halogen-bonded complexes between XY(ClF, BrCl, BrF) and H_nX(HF,H₂O and NH₃) were studied by supermolecular (SM) variational or SAPT (Symmetry Adapted Perturbation Theory) methods. The results display that the halogen-bonded complexes are more stable than the corresponding hydrogen-bonded complexes. The interaction energies of hydrogen-bonded and halogen-bonded complexes between XY(ClF, BrCl, BrF) and H_nX(HF,H₂O and NH₃) increase in the order BrCl < ClF < BrF. This order is correlated to the dipole moment of the XY. On the other hand, the energy decomposition shows that electrostatic and dispersion forces play an important role in the hydrogen-bonded complexes. For the halogen-bonded complex, the main interaction energy comes from the electrostatic energy. In the strongest N-X-type complex XY-NH₃, the induction energy is the most important attractive term, followed by the exchange and electrostatic energy. This domination of the induction and exchange terms is the main feature of the strong and partly covalent bonds.(2) The nature of the unusual hydrogen-bonded FCl-HX(HF,HClå’ŒHBr) and halogen-bonded complexes FCl-ClF formed by the interactions between two positively charged atoms of different molecules were studied by Atoms in Molecules (AIM) and SAPT methods. According to the SAPT analysis, induction and the dispersion energy play an important role in determining the equilibrium structure of these special complexes. AIM provides a very sophisticated analysis of the electron desity within a molecular system.(3) The nature of for the intermolecular interactions between furan and various hydrides H_nX(HF,H₂O> NH₃,HCl,H₂S and PH₃) were studied at the MP2/aug-cc-pVDZ level. Two types of geometry are observed in these interactions: theσ_o-type geometry (â… ), in which an H-O bond formed between a hydrogen atom of H_nX and the furan oxygen atom; TheÏ€-type geometry (â…¡) is characterized by aÏ€-H bond formed between a hydrogen atom of H_nX and theÏ€-electron system of the aromatic ring. The calculated interaction energies show that all of theσ_o-type complexes are more stable than the correspondingÏ€-type complexes for the first-row hydrides (HF,H₂O and NH₃). However, for the second row hydrides (HCl,H₂S and PH₃), all of theÏ€-type complexes are more stable than the correspondingσ_o-type complexes. To study the nature of the intermolecular interactions, an energy decomposition analysis was carried out and the results indicate that theσ_o-type complexes are traditional hydrogen-bonded complexes with electrostatic interactions making the primary contribution to complex formation. In the case of theÏ€-type complexes, the dispersion and electrostatic forces dominate complexation.(4) Equilibrium geometries, interaction energies and charge transfer for the intermolecular interactions between the thiophene and various hydrides H_nX(HF,H₂O,NH₃,HCl,H₂S and PH₃) were studied at the MP2/aug-cc-pVDZ level. Only one type of geometry is observed in these interactions: theÏ€-type geometry (â…¡). Although thiophene possesses both aÏ€-electron system and a non-bonding electron pair (n-pair), the geometries for all of the thiophene-H_nX complexes obtained in this study appear to be determined by theÏ€-electron system. By contrast, the furan-H_nX complex geometries were determined by both theÏ€-electron system and the n-pair. Furan and thiophene are both five-membered heteroaromatic rings with different heteroatoms.The molecular electrostatic potential maps of thiophene and furan, which might provide an insight into the disparity between the results obtained for furan and thiophene. When compared to the electrostatic potential of furan, it is apparent that the relatively weak electronegativity of sulfur confers a strong aromatic character on the thiophene ring. The electrostatic potential of the ring sulfur is insufficient to facilitate complex formation at the site of the S atom. The strongest negative electrostatic potential located at theÏ€-electron density near center of C₄-C₅. According to the SAPT analysis, the dispersion and electrostatic forces dominate theÏ€-type complexation.