Theoretical Design of Beryllium-based Planar Hypercoordinate Carbon Structures and Molecules with Ultrashort Be-Be Distances

Author:Zhao Xue Feng

Supervisor:wu yan bo


Degree Year:2019





Being the fourth element in the periodic table,beryllium(Be,1s22s2)is the smallest metal both as atom and as ion.It is difficult for Be to lose its two valence electrons entirely.Simultaneously,Be also exhibits the typical electron deficiency,so it often prefers the nonclassical delocalized multicenter bonding rather than classical localized two-center two-electron bonds.However,due to their high toxicity,the chemistry of beryllium compounds was not extensively explored,especially for the related experimental studies,which seriously hindered the beryllium chemistry from being rapidly developed.In this thesis,the preference for the multicenter bonding of beryllium was employed to design the species with planar hypercoordinate carbon,while the small size of beryllium was used for designing the species with beryllium-based ultrashort main group metal-metal distances(1.6921.895 ?).We are hoping that our high-accuracy calculation results can provide the feasible targets for experimentalist,thereby reducing the time that possibly exposuring to toxic beryllium compounds and thus promoting the rapid development of beryllium chemistry.The main contents are listed below:1.Design of beryllium-based nanomolecules with planar tetracoordinate carbonThis computational study identifies the rhombic D2h C2(BeH)4 to be a species featuring double planar tetracoordinate carbons(pt Cs).Aromaticity and the peripheral Be4 bonding around CC core contribute to the stabilization of the pt C structure.Although the such structure is not a global minimum,it possesses the high kinetic stability.Moreover,the electron deficiency of Be atoms allows self-assembly of Be2H2 hydrogen bridged bonds(HBBs)to link C2(BeH)4 units together to give nanostructures,including linear chains,planar sheets,and nanotubes.Formation of one HBB bond releases more than 30.0 kcal/mol of energy,implying the highly exothermic formation processes and the possibility to synthesize these nanosize structures.2.Theoretical design of planar pentacoordinate carbon species with berylliumThe diagonal relationship between beryllium and aluminum and the isoelectronic relationship between Be H unit and Al atom were utilized to design nine new planar and quasi-planar pentacoordinate carbon(pp C)species CAlnBemHx q(n + m = 5;q = 0,±1;x = q + m – 1)by attaching H atoms onto the Be atoms in CAl4 Be,CAl3Be2–,CAl2Be32–,and CAl Be43–.These pp C species are σ and π double aromatic.In comparison with their parents,these H-attached molecules are more stable electronically,as can be reflected by the more favourable alternative negative-positive-negative charge-arranging pattern and the less dispersed peripheral orbitals.Remarkably,seven of these nine molecules are global energy minima,in which four of them are kinetically stable,including CAl3Be2 H,CAl2Be3H–,CAl2Be3H2,and CAl Be4H4+.They are the promising targets for the experimental realization of speices with a pp C.3.Theoretical design of species with ultrashort Be-Be distancesWe computationally designed and characterized a series of novel main group species containing ultrashort metal-metal distances(1.7281.866 ?)between two beryllium atoms in different molecular environments,including a rhombic Be2X2(X = C,N)core,a vertical Be-Be axis in a 3D molecular star,and a horizontal Be-Be axis supported by N-heterocyclic carbene(NHC)ligands.The ultrashort Be-Be distances are achieved by affixing bridging atoms to attract the beryllium atoms electrostatically or covalently.Among these species are six global minima and one chemically viable diberyllium complex,which provide potential targets for experimental realization.4.Superalkali cations and superhalogen anions with ultrashort Be-Be distancesUsing the strong Lewis acid properties of Be2H3 structure,we had employed the Lewis base,including neutral L ligands and anionic X ligands,to design the superalkali cations [L→Be2H3←L]+(L = NH3,PH3,Ne,Ar,Kr and Xe)and supherhalogen anions [X-Be2H3-X]–(X = CH3,Cl,Br).The Ad NDP analysis of these structures shows that the non-classical 3c-2e σ bond is formed between Be-Be with the assistance of H atoms,which contributes to shorten the Be-Be distances(1.6921.826 ?).[Ne→Be2H3←Ne]+ is the first kinetically viable global minimum species with the main group metal-metal distance under 1.700 ?.[L→Be2H3←L]+(L = NH3,PH3,Ar,Kr,and Xe)and [L-Be2H3-L]–(L = CH3,Cl and Br)are first examples of kinetically stable global energy minima superalkali cations and superhalogen anions,respectively,with ultra-short metal-metal distances.