Designing Molecules With Ultrashort Be-Be Distances Based On The Isoelectronic Relationships Between C And Be₂ Moiety

The ultrashort metal-metal distance(USMMD,denoting the dM-M < 1.900 ?)is a concept proposed in combination with the studies on the transition metal quintuple bonds.The short character stressed on USMMD was employed to highlight the strong metal-metal bonding.Traditionally,the USMMDs were realized between transition metals due to their capability to support the homo-nuclear quintuple bond.Recently,our group had proved that the main group metal beryllium(Be),due to its small size and electron deficient properties,can support the USMMDs with the aid of proper auxiliary bridging atoms.A feasible way to achieve ultrashort Be-Be distance is the isoelectronic substitution of C atom in planar pentacoordinate carbon species CBe5H5+ with the Be2 moiety.In this thesis,such strategy was systematically explored,i.e.the C atoms in planar hexacoordinate carbon species and linear small organic molecules were substituted by Be2 moiety to achieve the new species with USMMDs.The works in the thesis can be divided into three parts:1.Design of Species with USMMDs Using Planar Hexacoordinate Carbon Structures as the Templates.On the top of our previous findings that isoelectronic substitution of C atom in planar pentacoordinate carbon species CBe5H5+,we tried to use planar hexacoordinate carbon species CN3Be3+ and CO3Li3+ as the template and in combination with the reasonable extension to design the Be2N3M3+(M = Be,Mg,Ca)and Be2O3M3+(M = Li,Na,K)species with the axial ultrashort Be-Be distances of 1.627–1.870 ?.The detailed electronic structure analyses suggested that the ultrashort Be-Be distances in these species were achieved by the favourable Coulomb attractions between the positively charged axial beryllium atoms and negatively charged bridging N or O atoms.In addition,the shorter axial Be-Be distances were determined firstly by the smaller size of bridging electronegative X(X = O,N)atoms and secondly by the lower electronegativity of peripheral M atoms,while the stability of newly designed species was closely related to the types of their valence electron pairs,where the localized two-center two-electron bonds were more desired for the stabilization than non-bonding valence lone pairs.In the newly designed species,Be2N3Be3+ and Be2N3Mg3+ were characterized to be the kinetically stable global minima,thereby providing the promising target for experimental realization of species with USMMDs between main group metals.2.Design of Species with USMMD Based on linear small organic molecules.Using species linear small organic molecules(HCCH,H2 CCO and HNCO)as the template and in combination with the reasonable extension,the HBe2 CH,OBe2CH2,OBe2 NH,HBe2C–,OBe2CH–,OBe2N– and HBe2-Be2 H species with the axial ultrashort Be-Be distances of 1.702–1.872 ? had been designed computationally.The detailed electronic structure analyses suggested that the ultrashort Be-Be distances in HBe2-Be2 H was achieved by the favourable covalent bonding within tetrahedral Be4 moiety,while those in other species were achieved by the favourable Coulomb attractions and covalent bonding character between the positively charged axial beryllium atoms and negatively charged bridging X(X = H,C,N and O)atoms.The majority of designed species(except for HBe2C–)are the kinetically stable global minima,which are the promising candidates for experimental realization.3.Elaborating the cationic Be5N3+ to achieve the anionic Be4N3– with USMMDs.In the first work of this thesis,the Be5N3+ species was characterized to be kinetically stable global minimum with USMMD.Nevertheless,the anionic species are more suitable for experimental realization in the negative anion photoelectron spectroscopy(PES).In current part,we had elaborated the Be5N3+ species by removing an axial Be atom and introducing two electrons to maintain the skeletal electrons,which resulted in the anionic species Be4N3– species.The Be-Be distances in this species are 1.853 ?,being the eligible USMMDs.The VDE of such anion was 3.70 e V,higher than the threshold of 3.62 e V for being regarded as superhalogen.It is remarkable that Be4N3– is the kinetically stable global minimum,which are the feasible target for experimental realization in PES.