Theoretical Studies On Bonds, Structures And Properties Of Intermolecular Interaction Systems

(1) By the counterpoise (CP) geometry optimization at the level ofCCSD(T)/aug-cc-pVDZ, the asymmetrical linear structures with all thereal frequencies were obtained for the triatomic cations of (ABA)⁺ type:Ne₃^<sup>+, (He-Ne-He)⁺, (Ar-Ne-Ar)⁺, (Ar-He-Ar)⁺, (He-O-He)⁺ and(Ar-O-Ar)⁺. The validity of this optimization method is confirmed bythe comparing with the method of the potential energy surface for thecalculations of Ne₃⁺ and (He-Ne-He)⁺. Using the molecular orbitaltheory, it is found that the interaction within the triatomic cations isdominated by the contribution from the first two atoms while thecontribution from the third atom is small. This result is justified as adirect consequence of forming an asymmetrical linear structure.Specifically, four types of interaction within the triatomic cations areidentified: three-electron σ-type hemibond, three-electron Ï€-typehemibond, two-electron σ bond and the attraction between cation andatoms. For Ne3+, (He-Ne-He)+ and (He-O-He)+ clusters, it is shown thatthe electron correlation effect supports the asymmetry.(2) Using ab initio calculation, eight structures with all realfrequencies are obtained for the Li2(HF)n (n = 1 4) clusters.According to the bonding characteristic of the Li2 subunit, thesestructures are classified into four kinds. Firstly, one structure includesthe Li-Li covalent bond with about distance of Li2. Secondly, threestructures include {Li-Li}+ hemibonds with about distance of Li2+.Thirdly, one structure has {Li Li}+ with van der Waals Li-Li distance.Fourthly, three structures have Li+ ···Li with more than van der WaalsLi-Li distance. The Li2(HF)n clusters, except cluster 4a with the centralsymmetry, are predicted to have the large first hyperpolarizabilities (β0)(1496 – 4700 au)) due to the existence of the diffuse electrons of Li2subunit. We find that increasing the number of HF molecule near Liatom with the large positive charge (atom Li2) or decreasing thenumber of HF molecule near Li atom with the small positive ornegative charge (atom Li1), the β0 value is enhanced. Thus the methodof increasing the β0 value is that decreasing the number of HF moleculenear Li atom with the small positive or negative charge (atom Li1) andincreasing the number of HF molecule near Li atom with the largepositive charge (atom Li2) in Li 2(HF) n (n = 1 4).(3) The structure and properties of the Li(HF)3Li cluster with C3hsymmetry are investigated using ab initio calculation. The Li(HF)3Li isa metal-nonmetal-metal sandwich-like cluster mainly connected by theintermolecular interactions. In the especial cluster, the (HF)3 containingthe triangular F ring with the negative charges is sandwiched inbetween two Li atom. It is interesting that under the action of thetriangular F ring with the negative charges, the valence electrons of twoLi atoms are pushed out to form the distended excess electron cloudthat surrounds the Li(HF)3Li as a core. So the Li(HF)3Li cluster showsnot only the electride characteristic but also new superatomcharacteristics. Some characteristics of the especial superatom arefound. (1) The superatom contains the double shell nucleus. Theinternal nucleus is the regular triangular ring made of three F atomswith the negative charge and the outer-shell nucleus is made of three Hand two Li atoms with the positive charge. (2) The bonding force of thissuperatom framework is mainly the intermolecular interaction force,the lithium bond, which is different from that (covalent bond or ionicbond) of the general superatoms. (3) The interaction between theouter-shell nucleus and the excess electron cloud is mainly theanti-excess-electron hydrogen bond. (4) The especial superatomexhibits the new aromaticity (NICS = -8.37 ppm at the center of theregular triangular F ring), which is the aromaticity found in the clusterof the intermolecular interaction. This is the new knowledge on thearomaticity. (5) The large polarizability of the superatom is revealed.Further, the vertical ionization energy (VIE) of the superatom is verylow, 4.51eV (smaller than 5.210eV of the alkaline-earth metal Ba) sothat it may be viewed as a super alkaline-earth metal atom.(4) Using ab initio calculation, three structures with all realfrequencies are obtained for the Li (HF)n (n = 1 4) clusters. Thedipole moments (μ), polarizabilities(α0) and the firsthyperpolarizabilities (β0) of Li(HF)n (n = 2 4) are investigated atMP2/6-311++G(3df,3pd) method. The large first hyperpolarizabilities(β0) (5572 11751 au) are found. It is revealed that the contributions ofLi polarized to α0 and β0 values are dominant.