| Brutschy and Hobza have pointed out the importance of weak interactions in the introduction of their paper [Chem. Rev. 100, 3862 (2000)] It is well known that these weak interactions play an important role in chemistry, physics, and particularly in the biodisciplines. Since all life on earth may be viewed as a matter of supramolecular chemistry, with vdW forces playing a central role. Discovering novel molecular complex with unusual properties and nature of new intermolecular interactions is always an exciting paper of chemistry. A great interest has been evinced in the recent past to examine their role on the structure and properties of molecular complexes. The theoretical studies were performed on the structures and properies of some representative systems containing weak interactions in this thesis. The results obtained on new structure and intermolecular interactions may be valuable for improving our understanding of the nature of intermolecular interaction in biological molecular recognition, crystal packing, cluster formation, and so on.(1)By means of ab initio calculations of MP2/6-311+G(2d,2p) method, the optimized geometry of the CH2F2…H2O complex is obtained. The structure of a subunit in the complex is similar to that of its molecule (CH2F2, H2O). It was found that the geometry structure of the complex CH2F2…H2O is close to its experimental structure. ∠(F…H-O)=116.5°is large deviation from a strictly linear H-bond. It is a result that a strong π-type secondary hydrogen bond is formed. Using a probing point charge to detect the orientations of the lone pairs at atom O of the CH2F2…H2O, we establish here aπ-type secondary H-bond interaction model because long pairs ni and CHi(i=1,2) are almost parallel and in a plane, the additional secondary H-bond between ni and CHi(i=1,2) form. The doubleπ-type H-bond is very strong. Furthermore, the stabilized energy of bending H-bond comes from π-type H-bond (= -0.0658 ev) is also calculated. (2) The research about the new type non-linear optical materials have shown us a new field to investigate the interactions between molecules. The character of the crystal material owns not only to the molecule itself, but also to the arranged mode and the interactions between molecules. So the research about the systems with intermolecular interactions on the non-linear optical character will lead to the design on non-linear optical materials from molecule level to real materials. Many theoretical research on this filed have been carried out. By means of ab inito calculations of MP2/aug-cc-pVDZ and B3LYP/aug-cc-pVDZ methods, the optimized geometries of the (HF)n (n=1~10) are obtained. The (HF)n structure is approach to HF chain configuration in crystal, when number of the HF molecule is large. The static dipole moments (μ0), polarizabilities (α0) and first hyperpolarizabilities (β0) of (HF)n (n=1~10) clusters are investigated by means of MP2, B3LYP and HF method with basis sets aug-cc-pVDZ.(3) By counterpoise-correlated potential energy surface method (interaction energy optimization), the structure of the ( H–bond complex FH…C4H4…HF has been obtained at the second-order M?ller-Plesset perturbation (MP2/aug-cc-pVDZ) level. Intermolecular interaction energy of the complex is calculated to be -7.8 kcal/mol at the coupled-cluster theory with single, double substitutions and perturbatively linked triple excitations CCSD (T)/ aug-cc-pVDZ level. The optimized structure is a "wheel with a pair of pedals" shaped (1(1) structure in which both HF molecules almost lie on either vertical line passing through the middle-point of the C = C bond on either side of the horizontal plane of the C4 ring for cyclobutadiene. In the structure, an antiaromatic ring (- dihydrogen bond is found, in which the proton acceptor is antiaromatic 4 electron and 4 center (-bond and the donors are both acidic H atoms of HF molecules. In accompanying with the (- dihydrogen bond, two secondary interactions are exposed. The first is a repulsive interaction between an H...
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