Theoretical Studies On Bonding, Structures And Properties Of Alkali Metal-containing Systems

In this thesis, several types of alkali metal-containing systems aretheoretically investigated., Their bonding natures, structures and properties arediscussed in detail. The investigated alkali metal-containing systems includelithium bond dimers C2H4-nFn···LiH (n=0,1,2), sodium bond dimersXH···NaH (X=HBe, LiBe, NaBe, HMg, LiMg, and NaMg), a hexanuclearalkali metal π-coordination compounds [18]Annulene-Li6-[18]Annulene with[18]Annulene as face-capping ligands, a new type of endohedralmetallofullerene-superhalogen compound, Li@C60-BX4(X=F, Cl, Br). Themain contributions are as followings:1. Using the second-order M ller-Plesset perturbation method, thestructures and π-lithium bonding properties of C2H4-nFn···LiH (n=0,1,2)dimers were analyzed. The results showed that F substitution led to π electroncloud deformation in the ethylene molecule, and subsequent changes(deviation, elongation, and bend) in the π-lithium bonds in C2H4-nFn···LiH. Incontrast to the π-hydrogen bonds in the C2H4-nFn···HF (n=0,1,2) systems, theπ-lithium bonds in the C2H4-nFn···LiH dimers were obviously bent because ofsecondary hydrogen bond interactions, and they exhibited weak directivity.For the four C2H4-nFn···LiH dimers, the strength of the interaction was-8.09kcal·mol-1(C2H4-LiH)>-6.53kcal·mol-1(C2H3F-LiH)>-5.10kcal·mol-1 (cis-C2H2F2-LiH)>-4.84kcal·mol-1(g-C2H2F2-LiH) at theCCSD(T)/6-311++G(3df,3pd) level. This indicates that the F substituenteffect decreases the strength of the interaction between ethylene and LiHmolecules.2. A new kind of sodium bonding complexes XH···NaH (X=HBe, LiBe,NaBe, HMg, LiMg, and NaMg) have been predicted and characterized in thepresent paper. For each XH···NaH complex, the hydride-sodium bond isformed between the negatively charged H atom of XH and the positivelycharged Na atom of NaH. Due to the formation of the complexes, both theX-H and the Na-H bonds are elongated, and the Na-H stretching vibrationalfrequency is redshifted. The interaction energies in the XH···NaH complexesat the MP2/6-311++G(3df,3pd) level increased in the order: HBeH···NaH(-4.50kcal/mol)<HMgH···NaH (-8.69kcal/mol)<LiBeH···NaH (-11.07kcal/mol)<NaBeH···NaH (-11.38kcal/mol)<LiMgH···NaH (-14.90kcal/mol)<NaMgH···NaH (-15.35kcal/mol). It is found that, the more metallicity of theX part in the XH molecule, the stronger sodium bond interaction between XHand NaH. By comparisons with some related systems, it is concluded that thestrength of hydridic H bond is increased in the order: dihydrogen bond <hydride-sodium bond <lithium-hydride lithium bond.3. By means of density functional theory, a hexanuclear sandwichcomplex [18]Annulene-Li6-[18]Annulene which consists of central Li6hexagon ring and large face-capping ligands,[18]Annulene, is designed andinvestigated. The large interaction energy and HOMO-LUMO gap suggeststhat this novel charge-separated complex is highly stable and may beexperimentally synthesized. In addition, the stability found in the [18]Annulene-Li6-[18]Annulene complex extends to multidecker sandwichclusters (Li6)n([18]Annulene)n+1(n=2-3). The energy gain upon addition of a[18]Annulene-Li6unit to (Li6)n-1([18]Annulene)nis pretty large (96.97~98.22kcal/mol), indicating that even larger multideckers will also be very stable.Similar to ferrocene, such hexanuclear sandwich complex could be consideredas a versatile building block to find potential applications in different areas ofchemistry, such as nano-science and material science.4. It has recently been demonstrated that superatoms which can exhibitbehaviors reminiscent of atoms in the periodic table might have syntheticutility, and represent potential building blocks for the assembly of novel,nanostructured materials [Science2004,304,84-87; Science2005,307,231-235; J. Phys. Chem. C2009,113,2664]. In this work, a new type ofendohedral metallofullerene-superhalogen compound, Li@C60-BX4(X=F, Cl,Br), is theoretically constructed and characterized by using the densityfunctional theory. The electron transfer from Li@C60to BX4contributesgreatly to the Li@C60-BX4compound formation. Such compounds exhibitconsiderable stabilities with large binding energies, ionization potentials, aswell as HOMO-LUMO gaps. The investigation on nonlinear optical (NLO)properties of Li@C60-BX4reveals the strong dependence of the static firsthyperpolarizability β0on the atomic number of the involved halogen atom X,which means that one can enhance the first hyperpolarizabilities of theendohedral metallofullerene upon combination with proper superhalogen. Thepresent investigation may promote the development of novel nanomaterialswith unusual properties (i. e. NLO properties), and enrich the knowledge of chemical bonds (for example, long-range interaction between trapped atom inC60cage and the outside superatom motif).