Theoretical Investigations On The Binary Boron-rich Clusters

The chemical properties of boron lie between the metal and nonmetal. Boron can either react with the former to give birth to metal borides or react with the latter to bring nonmetal borides. Boron and boron compounds are widely used in modern life, such as materials, anti-tumor therapy, high-temperature superconductors, nanotechnology and other industrial, agricultural, national defense, cutting-edge scientific fields. Most of boron and boron compounds are high-temperature resistant, wear resistant and have other unique physical structure and chemical properties, which aroused many chemists'great interest. In this thesis, detailed quantum chemical investigations on the potential energy surfaces of a series of important binary boron-rich clusters have been carried out. The results obtained from the potential energy surfaces lead us to understand the detailed isomerism between various isomers, and will be helpful for understanding the doping and oxidation processes of pure boron clusters kinetically. These molecules with high thermodynamic and kinetic stability may be used as building cores in the growth of boron-oxide clusters. Our investigations will provide reference for the growth process and oxidation mechanism of boron-oxidation cluster and boron-nitride cluster, and will be beneficial to the future experimental characterization. The main results are summarized as follows:1) Despite the fact that B₃O is the second simplest BnO radical after BO, a controversy recently emerged concerning the molecular structure of its global minimum. Two recent theoretical groups predicted the linear quartet BBBO to be the ground isomer. By contrast, another recent theoretical group reported that B₃O has a doublet B₃-ring ground structure. Moreover, larger B_nO clusters usually have low-lying B₃-ring isomers. In order to provide the accurate energetic competition between linear and cyclic structures in both doublet and quartet, and to understand the detailed isomerism between various isomers, which is vital for understanding the formation mechanism of B₃O, we report the first potential energy surface (PES) study of B₃O at various computational levels including CCSD(T)/6-311+G(2df), CCSD(T)/aug-cc-pVTZ, CCSD(T)/aug-cc-pVQZ and G3B3 for single-point energy as well as the B3LYP/6-311+G(d) and QCISD/6-311+G(d) for geometrical optimization. It is shown that the isomers in quartet state are all thermodynamically more stable than the corresponding doublet ones, and on both the quartet and doublet PESs, the linear form has the lowest energy. The dissociation energies for direct combination processes B₃+O, B₂+BO and B+B₂O are discussed, and the great discrepancy on the ground state structures of B₃O between the two recent reports is solved.2) Using CCSD(T)/6-311+G(2df)//B3LYP/6-311+G(d) method, we systematically investigated the structures and energetics of various B₄O isomers as well as the dissociation and isomerization stability of important species. It was shown that the singlet planar triangle structure containing the–BO unit ¹01 has the lowest energy, followed by the belt-like B₄O isomer ¹02 (2.6 kcal/mol). Both ¹01 and ¹02 possess good kinetic stability, and thus might have potential for existence. However, the previously reported triplet linear isomer BBBBO (34.9 kcal/mol) lies energetically much higher than the isomers ¹01 and ¹02.3) Boron-centered radicals have received growing interest. Recently, two groups reported density functional theory (DFT) investigations (GGA-PW91 and B3LYP) on a hexa-atomic boron-oxide radical, B₅O, which has led to great discrepancies on the type of low-lying structures. In this paper, we not only explore the energetics of doublet and quartet B₅O isomers at high electron-correlated levels (CCSD(T)/6-311+G(2df), CCSD(T)/aug-cc-pVTZ and G3B3), but also investigate the isomerization and fragmentation stability of the low-lying B₅O isomers. All the high-level studies consistently show that the B₅O radical possesses a belt-like ground structure ²01 in doublet electronic state followed by a knife-like isomer ²02 with an exocyclic–BO moiety at around 3.0 kcal/mol. The high thermodynamic and kinetic stability of ²01 and 202 might make them as important building cores in the growth of boron-oxide clusters.4) CB₆^2-, a 6Ï€-electron system with a planar hexa-coordinated carbon center, was first theoretically designed in ²000. Since then, a large number of publications have focused on CB₆^2- and related molecular species with hyper-coordinated centers. These species have been given a vivid name"molecular wheel", and at present wheel-like isomers with B centers are synthesized successfully in laboratory, it is predicted to be as an ideal building block to synthesize more complex compounds. We choose to study a sept-atomic boron-rich binary cluster OB₆, which is isoelectronic to the milestone molecule CB₆^2-. We performed a detailed potential energy surface survey of OB₆ covering various isomeric forms at the B3LYP/6-311+G(d) level. The planar knife-like isomer with a–BO moiety has the lowest energy, followed by the planar belt-like isomer. Three isomers (01, 02 and 05) have good kinetic stability, rendering their possible isolation. The remaining isomers have much lower conversion barriers towards isomerization to lower-lying forms. It is hopeless to detect any B or O-centered hexa-coordinated molecular wheel OB₆. The great discrepancy on the ground state structures of B₆O between the two recent reports is solved in our reports.5) We investigated the structures and energetics of various B₇O, B₈O, B₉O, B₁₀O, B₁₁O, B₁₂O isomers and compared with the isomers of B₄O, B₅O, B₆O. Our study shows during the growth process of boron-oxide cluster, two structures (knife-like form with exocyclic BO moiety and belt-like form) are competitive beginning with B₄O. With the increase of the number of boron atoms, the wheel form molecules also take part in the competition, and the B-centered molecular wheels become to the ground state structures begin with B₇O.6) Through investigating the detailed potential surfaces survey of the B₃O⁺, B₄O⁺, B₅O⁺ and B₆O⁺ cations, we found that the global minima are the linear singlet BBOB for B₃O⁺, belt-like for B₄O⁺, B₅O⁺ and B₆O⁺, and wheel-like for B₇O⁺. The growth pattern of boron-oxygen clusters and bare boron clusters are very similar, according to the relationship of both ground state structures. We speculate that the ground state structures of larger boron-oxygen clusters will appear multi-center structure or columnar structures.7) NB6- is isoelectronic to the milestone molecule CB₆^2- and it has been theoretically reported in the literature as a structural unit in sandwich compounds, such as [(h6-B₆X)₂M] (X=C, N; M=Mn, Fe, Co, Ni),(CnHn)M(B6X)M (CnHn) (M= Fe, Ru, Mn, Re; X = B, C, N; n = 5, 6), (B₆X)₂M (X = C, N; M = Ru, Rh, Pd, Os, Ir, Pt) and some half-sandwich compounds. However, there has been no research about the structure and stability of its own issue. In this paper, we carried out a detailed study on the isomerization of NB₆^- and found that the ground-state structure possesses the belt-like form. And the molecular of nitrogen centers is not only thermodynamically unstable, but also extremely kinetically unstable. By contrast, in the CB₆^2- system, although the carbon center of wheel-like structure is thermodynamically unstable, but it is kinetically stable.8) FB₆⁺ is isoelectronic to the molecules CB₆^2-, NB6- and B₆O. In this article we built the potential energy surface of FB₆⁺. The results showed that the ground state is a knife-like structure containing–BF which is similar to the ground state structure of exist and there was no belt-like structure .