Theoretical Studies On The Structure And Properties Of Fluxional Boron Alloy Clusters

In 2010,the discovery of the first "Wanker" molecular motor B19-cluster opened the prelude of the research of boron-based clusters molecular machines.Subsequently,the design,synthesis,and characterization of boron-based clusters with dynamic fluxional have attracted widespread attention from researchers in the fields of chemistry and materials science,becoming one of the research focuses.Based on the research of the Mg2B8 nano-compass in the literature,we have theoretically designed a series of new boron alloy nano-compass M2B7-(M=Zn,Cd)and Mg2BeB8 by modifying the components(needle,dial)in this thesis.By introducing a transition metal tungsten atom,the rotation barrier between the inner and outer rings of the B10 cluster is diminished,and a new boron alloy Wanker molecular motor B10W is designed.Based on theoretical design,we use density function theory(DFT)to systematically study the geometric structures,chemical bonding,aromaticity,and dynamical behavior of these novel boron alloy clusters in this thesis.The results have further enriched and expanded the field of boron alloy clusters molecular machines The main research contents and conclusions of this paper are as follows1.Binary boron alloy nano-compass M2B7-(M=Zn,Cd)Using Mg2 and the B8(B(?)B7)molecular wheel containing seven-coordinated boron used as the needle and dial,respectively,MgZBS has been reported as a nano-compass in the literature.Using transition metals Zn and Cd to replace the Mg atoms of the nano-compass Mg2B8,and a B7(B(?)B6)molecular wheel as the dial,we have designed a binary boron alloy nano-compass M2B7-(M=Zn,Cd).The systematic isomer search and high-level quantification calculation show that the boron alloy nano-compass M2B7-(M=Zn,Cd)is the global minimum on the potential energy surfaces,which has good thermodynamic stability.Born-Oppenheimer molecular dynamics(BOMD)simulations reveal that the M2B7-(M=Zn,Cd)clusters have novel dynamic fluxional at 600K.The Zn2/Cd2 needles floating above the B7 dial can rotation freely,like a compass.Bonding analysis shows that there are both ionic bonding and covalent bonding between the Zn2/CdZ unit and the B7 dial in the M2B7-(M=Zn,Cd)binary boron alloy clusters.There are delocalized 6π and 6σ electrons in M2B7-(M=Zn,Cd)clusters,which make them possess the 6π/6σ double aromaticity according to the Huckel(4n+2,n=1)rule.The design of the new compass M2B7-(M=Zn,Cd)indicates that changing the dial and needle is an effective way to design the new boron alloy nano-compasses2.Ternary boron alloy nano-compass Mg2BeB8Based on the Mg2B8 binary nano-compass,we have theoretically designed the first ternary boron alloy nano-compass Mg2BeB8 by doping Be atoms.The Mg2 unit and the BeB8 eight-membered ring molecular wheel are the needle and dial,respectively,in the Mg2BeB8,which possesses a half sandwich-like structure.The global structure search shows that the Mg2BeB8 nano-compass is the global minimum structure on the potential energy surfaces,which possesses the good thermal stability Dynamics simulations show that the Mg2 needle can float freely above the BeB8 dial at 300K.The rotational energy barrier is only 0.1 kcal·mol-1 at the CCSD(T)/6-311+G(d)//PBE0/6-311+G(d)level.Chemical bonding analysis shows that Mg2BeB8 ternary boron alloy cluster is essentially regarded as charge transfer complex[Mg2]2+[BeB8]2-.There is a covalent Mg-Mg bond in the[Mg2]2+ needle.The dial[BeB8]2-has three delocalized π bonds and three delocalized a bonds,making it possesses the 6π+6σ double aromaticity.The ionic bonding between the[Mg2]2+needle and the dial[BeB8]2-and the continuously delocalized electron cloud make the Mg2BeB8 cluster has good fluxionality behavior.Compared with the seven-member ring dial of Mg2B8 nano-compass,the eight-member ring dial Mg2BeB8 has the better directivity.3.Boron alloy Wankel molecular motor WB10We have theoretically designed a boron alloy Wankel molecular motor WB10 by introducing a transition metal W(tungsten)atom to the elongated B10 bare boron cluster.The global search shows that the WB10 molecular motor is the global minimum(GM)on the potential energy surfaces.The GM possesses a bowl-shaped half-sandwich structure with C2v symmetry,in which the tungsten atom is located above the concave surface of the bowl-shaped B10 unit.Vibration frequencies analysis shows that the WB10 cluster has a soft vibration mode of 204.3 cm-1,which is related to the intramolecular rotation.A transition state(TS)structure can be found along the soft vibration mode.The introduction of the W atom can significantly diminish the energy barrier for the relative rotation of the inner and outer rings of the B10 unit.The rotational energy barrier is only 1.56 kcal·mol-1 at the CCSD(T)/def2-TZVP//PBE0/def2-TZVP level.Molecular dynamics simulations show that the WB10 cluster can be regarded as a Wankel molecular motor at 600 K:the peripheral B8 ring can rotate freely around the B2 core.Bonding analysis shows that,eighteen electrons are used for eight B-B single bonds on the peripheral B8 ring and one B-B single bond for the central B2 unit in the WB10 cluster.The remaining eighteen valence electrons participate in the formation of six delocalized a and three delocalized π bonds.Thus,WB10 cluster possesses the 6π aromaiticy and 12 σantiaromaticy,according to the Huckel rule.