Theoretical Studies On The Intermolecular Interactions For Some Typical Systems

The research included in this thesis spans subjects ranging from blue-shifting halogen bond to the world of small hydrogen-bonded water and glycine clusters and to the van der Waals interactions. The main results are as follows:(1) Geometries, harmonic vibrational frequencies and interaction energies of the water-hydrogen sulfide dimer, hydrogen fluoride dimer and glycine zwitterion-water dimer were investigated by the counterpoise-corrected (CP-corrected) gradient optimization that explicitly corrects for the basis set superposition error (BSSE) and CP-uncorrected (normal) gradient optimization at the B3LYP and MP2 levels of theory, respectively,employing the popular Pople's standard 6-31G(d), 6-31G(d,p) and 6-311++G(d,p) basis sets in order to assess the importance of CP-corrected gradient optimization in the study of hydrogen bonded systems. The normal optimization of these three H-bonded systems obtained using these popular basis sets all yielded erratic results, whereas use of CP-corrected gradient optimization led to consistent results with those from larger basis sets. So this CP receipt becomes useful and necessary to correctly describe large systems, where the use of small basis sets may be necessary. At the same time, the importance of convergence criterion for geometry optimization was also investigated using PH3???H2O model complex.(2) A new type of intermolecular bond, termed a blue-shifting halogen bond, is found in the chlorotrifluoromethane, bromotrifluoromethane, chlorotrifluorosilicane and chlorodifluoroamine related complexes. Counterpoise-corrected gradient optimization performed at a correlated ab initio level (MP2(full)/6-311++G(d,p)) shows a shortening of the C-Cl (C-Br, Si-Cl, or N-Cl) bond of the proton donor and a blue-shifting of the corresponding C-Cl (C-Br, Si-Cl, or N-Cl) stretching frequency. In contrast to the conventional hydrogen bond and the blue-shifting hydrogen bond, the topological and electronic properties and the origin of blue-shifting halogen bond are also investigated. (3) The structures, interaction energies, electronic properties for different conformers of glycine(Ip)-water complex have been determined employing density functional theory (DFT) using the B3LYP hybrid exchange-correlation functional with the Pople's standard basis sets. Four local minima and six transition states were obtained at the B3LYP/6-311++G(3d,3p) level of theory on the potential energy surface (PES) of glycine(Ip)-water complex. The results indicated that the cyclic double hydrogen bonding conformer involving the –COO- group and water is the global minimum on the PES. All the data clearly show that it is a strong hydrogen bonding complex and should be observed experimentally. The DFT methods showed good agreement in reproducing the geometrical parameters and the energetics compared to the MP2 results. The time consuming CP-corrected gradient optimization is not necessary for the study of such strong hydrogen-bonded complex.(4) Computations are presented for the glycine-(H2O)n (n=1–2) and its zwitterion-(H2O)n (n=1-2) clusters. We find that at least two water molecules need to bind to glycine to give stable glycine zwitterion-water cluster. Structures of the conformers are predicted, and their relative energies are compared. Detailed analysis is presented on the dynamic (proton transfer) pathways between the neutral glycine-(H2O)2 and the zwitterionic glycine-(H2O)2 clusters, including the structures of the transition states. A variety of proton transfer pathways are predicted between glycine-(H2O)2 and the zwitterion-(H2O)2 clusters, depending on their structures: direct proton transfer, concerted double proton transfer mechanism.(5) The P?H???P interactions were investigated using a model complex phophine dimmer with its three minimum energy conformers on the counterpoise corrected potential energy surface. At the MP2(full)/6-311++G(3df,3pd) level, the basis set superposion error corrected interaction energy of the three minimum energy conformers is -1.26, -0.8...