Theoretical Design Of Cyclic Be-H Clusters And Double-chain Be-H Nanoribbon With Planar Hypercoordinate Bonding Geometries

Boron and beryllium are typical electron-deficient elements.The recent studies indicated that boron can form the cyclic ring structures with centripetal bonding ability.This feature make it potentially useful for designing the highly symmetric molecules with planar hypercoordinate center atom.In contrast,beryllium itself does not possess enough valence electrons and can not form the cyclic structure having similar bonding behaviors.In the recent studies of our group,we designed a planar pentacoordinate carbon（ppC）molecule Be₅H₅C+(C_5v)and planar hexacoordinate boron（phB）molecule Be₆H₆B+（D6h）.The detailed electronic structure analyses suggest that each beryllium and hydrogen in Be₅H₅C+ and Be₆H₆B+ use one electron to form the peripheral three-center two electron（3c-2e）bonds;and the remained electron of beryllium can be used to interact with center pp C or phB.Such centripetal bonding behaviors of Be₅H₅ and Be₆H₆ rings are very similar to those of boron rings.In this work,based on such findings,we extensively explored the possibility of using beryllium-hydrogen rings to design the planar hypercoordinate structures.We found that beryllium-hydrogen rings are more suitable for designing the planar hypercoordinate structures around the atoms,which are large in size but need relatively less number of centripetal electrons.The molecules designed possess good thermodynamic and kinetic stabilities,which are promising for the experimental realization.In this thesis,our attentions had been paid to two related systems,whose contents and conclusions are briefly introduced below:1.Aromatic Be-H Stars Enclosing Planar Heptacoordinate Late Transition MetalsBe₅H₅ and Be₆H₆ rings are suitable for accommodating a carbon or boron at their center to give a stable 8-electron shell structure.In this thesis,we explored the possibility of using Be₇H₇ ring to design the planar heptacoordinate atoms.We found that such ring fit group 11 elements（Cu,Ag,Au）to design the neutral star-like species Be₇H₇M（M =Cu,Ag,Au）with stable 18-electron shell structure around the metal.Simultaneously,the group 11 elements can be replaced by group 10 or group 12 elements,forming the anionic or cationic species with planar heptacoordinate metals.These star-like species are stabilized by the favourable sandwich type “–+–” charge distribution,the stable eight electron shell structure,and the σ and π double aromaticity.Among these species are three thermodynamically stable species（Be₇H₇Cu,Be₇H₇ Au,Be₇H₇Pd-）,in which two of them（Be₇H₇Cu,Be₇H₇Au）are kinetically stable.Be₇H₇ Cu and Be₇H₇ Au represent the first examples global minima with p7 Ms structure for late transition metals.Hopefully,these interesting species can be experimentally realized in future.2.Zigzag Double-Chain C-Be Nanoribbon Featuring Planar Pentacoordinate Carbon and Ribbon AromaticityMost planar hypercoordinate carbon atoms in the low-dimensional material（LDMs）are planar tetracoordinate carbon,while the examples in which the carbon atom adopts higher number of coordination are extremely rare and the bonding geometries of those carbons are not perfectly planar.Via the rational selection of edge-shared positions in pentagon for fusions,we use ppC molecule CBe₅H₄,in this thesis,as building block to design the ribbon-like clusters CnBe3n+2H2n+22+（n = 2–5）with planar pentacoordinate carbons（ppCs）and extend the ppC bonding pattern to zigzag double-chain C-Be nanoribbon considering 1D periodic boundary condition.The carbon atoms in the nanoribbon adopt the perfect planar penta-coordination,which is unprecedented in low-dimensional materials.Geometrically,it is important for achieving the perfect ppC to open the Be-Be edge without H on the Be5 ring in the nanoribbon.Electronically,the perfect ppC structures are stabilized by the favourable sandwich charge distribution,the σ and π double aromaticity（indicated by the results of natural density distribution（AdNDP）and nuclear independent chemical shift（NICS）analyses）,and the satisfactory of the octet rule for ppCs.Interestingly,the delocalization of σ and π electrons are limited in the CBe5 moiety,which are not seen in the low dimensional materials with fused-rings.Nanoribbon and its molecular precursors are thermodynamically stable and are promising targets for experimental realization.The nanoribbon is predicted to be the indirect band gap semiconductor and it should be useful in designing the light weight electronic devices.