Investigations On Boron-based Nanoclusters And Fluxional Bonds In Chemistry

Nanomaterials have wide applications in chemistry,physics,biology,medicine,materials,and nanodevices due to their small particle sizes,large specific surface areas,and high surface energies.Progresses in carbon nanomaterials in the past four decades have promoted the rapid developments of nanoscience and technology.Because of the obvious similarities and differences between boron and carbon,boron nanomaterials and boron-carbon binary nanomaterials have aroused much attention consequently.Boron nanomaterials and boron-carbon binary nanomaterials exhibit interesting structural,bonding patterns and physical and chemical properties due to the prototypical electron-deficiency of boron.L.S.Wang and co-workers have successfully revealed the structural and bonding characteristics of anionic boron clusters Bn-/0 in the size range with n=3-42,48 through joint photoelectron spectroscopy(PES)and first-principles theory investigations in the past two decades.But the bonding nature of the observed fluxional boron nanoclusters B19-and B39-still remains unknown to date.Theoretical and experimental studies also found that metal-doping induces dramatic structural and bonding changes to boron nanomaterials,forming diversified boron nanomaterials.To reveal the bonding nature of experimentally observed fluxional B19-,Ta@B20-,B39-,and B182-,based on extensive first-principles theory calcualtions and detailed AdNDP bonding analyses,we proposed in this work the concept of fluxional bonds(FBs)in chemistry which form and break constantly under certain consitions.FBs have also been employed to investigate the fluxional behaviors of the newly proposed half-sandwich MB18-(M=K,Rb and Cs).Based on the similarities and differences between B and C,we have predicted a series of novel B-C binary fullerenes and graphenes,and explored their geometric structure and bonding pattern at the density functional theory level.The main results are summarized as follows:1.Fluxional bonds in planar B19-,tubular Ta@B20-,and cage-like B39-Based on the first-principles theory calculations and AdNDP bonding analyses,we found the planar B19-has five multicenter fluxional ? bonds between the inner B6 pentagon and outer B13 ring which form and break constantly,while the tubular Ta@B20-has 2 multicenter fluxional ? bonds between the B2 bar on the top and the[Ta@B18]-double-ring at the bottom and the cage-like B39 has 3 multicenter fluxional ? bonds at the three B5 active sites to faciliate the W-X-M transformation.The structural fluctuations in B19-,Ta@B20-,and B39-between ground states and transition states originate from the multicenter fluxional bonds(FBs)in the systems which form and break constantly under the certain conditions in concerted mechanisms.FBs are an extension of the classical localized bonds and delocalized bonds in chemistry.2.Fluxional bonds in planar B182-and half-sandwich MB18-(M=K,Rb and Cs)Based on extensive global minimum searches and the density functional theory investigation,we predict a series of half-sandwich MB18-(M=K,Rb and Cs)which forming typical charge-transfer complexes.We systematically studied the geometric structure,chemical bonding,and dynamic fluxionalities of B182-and MB18-(M=K,Rb and Cs)and found that B182-and MB18-(M=K,Rb and Cs)exhibit pseudo-rotations between B12 outer ring and B5 inner ring at 400K,corresponding to a soft vibration mode associated to the rotation between the B5 inner ring and the B12 outer ring.Bonding analysis shows that the five multicenter ? bonds and ? bonds between the inner ring of B5 and the outer ring of B12 are constantly breaking and forming,which facilate the fluxional behaviors of the B182-and MB18-(M=K,Rb and Cs).These multi-center ? bonds and ? bonds are typical fluxional bonds.3.Cage-like B40C30,B40C40,and B40C50:high-symmetry heterofullerenes isovalent with C60,C70,and C80The discovery of the borospherenes family Bnq(n=36-42,q=n-40)presents the possibility to form B-C binary heterofullerenes with the dual structural characteristics of borospherenes and fullerenes.Based on global minimum searches and first-principles theory calculations,we predict three cake-like B-C binary heterofullerenes S10 B40C30,C5 B40C40,and S10 B40C50 which are isovalent with C60,C70,and C80,respectively.These B-C binary heterofullerenes feature one B30 boron double chain on the waist,and two C15 or C25 caps on the top and bottom,inheriting the characteristics of boron double chains(BDCs)and carbon single chains(CSCs).Detailed orbital and bonding analyses indicate that all these heterofullerenes contain 20 delocalized 5c-2e ?-bonds in ten hexagonal pyramids that are evenly distributed around the waist.Such structural and bonding patterns render high stability to these B-C heterofullerenes.4.Novel B-C binary fullerenes following the isolated B4C3 hexagonal pyramid ruleBased on the B4C3 semiconducting boron carbide monolayer with isovalent to graphene,we predict a new class of B-C binary heterofullerenes following the isolated B4C3 hexagonal pyramid rule.These binary spherically aromatic heterofullerenes C5h B20C35,C5 B20C45,C5h B20C55 and C5 B20C65 which are isovalent with C50,C60,C70 and C80,respectively,possess five isolated B4C3 hexagonal pyramids distributed on the waist,while S10 B40C50,C5 B40C60,S10 B40C70 and C5 B40C80 isovalent to C80,C90,C100 and C110,respectively,possess ten isolated B4C3 hexagonal pyramid on the cage surface.Detailed bonding analyses show that all of these B-C binary heterofullerenes follow similar bonding patterns with their isovalent fullerene analogues,with three delocalized 7c-2e ? bonds over each isolated B4C3 hexagonal pyramid,forming a local ?-aromatic system.The formation energies of the(B4C3)nC60-6n(n=1-5)series appear to increase almost linearly with the number of isolated B4C3 pyramids in the system,indicating that B4C3 hexagonal pyramids are independent.5.B-C binary fullerenes and graphenes following the isolated B4C3 hexagonal pyramid ruleBased on extensive first-principles theory calculations and B4C3-C6 analogy,we present the possible existence of S6 B48C48 binary fullerene constructed from D3d C84 with B:C=1:1.This highly stable B-C binary fullerene possesses twelve isolated B4C3 pyramids evenly distributed on the cage surface,matching the isolated B4C3 hexagonal pyramid rule,with each B4C3 hexagonal pyramid possessing 3 7c-2e ?-bonds to form a 4n+2 local aromatic system(n=1).Based on the structural characteristics of S6 B48C48,we designed a series of B-C binary graphenes composed of isolated B4C3 pyramids with B:C=1:1.The most stable P1 B25C25 turns out to be a typical anisotropic two-dimensional conductor,being metallic in y direction and semiconducting in the x direction,while the second lowest-lying P1 B24C24 appears to be a direct-gap semiconductor with the band gap of ?Egap=1.61 eV.Armchair and zigzag B-C binary nanotubes obtained from B25C25 are semiconducting when rolled up in x direction and metallic when rolled up in y direction.