Theoretical Study Of Energy Gaps And Electronic Structures Of 14e And 16e Transition Metal-capped Carbon Chains

Transition metal capped linear carbon chain?MCC?is composed of sp-hybridized carbon atoms,transition metal termini and ancillary ligands.This linear structure makes MCC show excellent strength,long-range conductivity and special photoelectric properties.It has potential applications in molecular wire,molecular magnets,photoelectric device and non-linear optics.Due to the experimental difficulties,theoretical investigations utilizing density functional theory?DFT?have been done to three 16-electron?16e?and two14-electron MCCs in current work.Mean bond length alternation?MBLA?,singlet-triplet energy(?E_S-T)splittings?STES?,HOMO-LUMO gap and electronic structures will be discussed including the following contents.?1?All the three 16e MCCs,Ru CC,Mn CC,and W¹CC,exhibit odd-even splittings and alternations of the?E_S-T,HOMO-LUMO gap,MBLA and metal's electron back donation when increasing carbon chain?CC?length.These alternations indicate great difference in odd and even MCCs.The frontier molecular orbitals?FMO?are non-degenerate in odd MCCs and near-degenerate in even MCCs.Such electronic structures lead to noticeable difference of?E_S-T and other properties in odd and even MCCs.Mn CC and Ru CC show large splittings,where the FMOs are primarily contributed by the transition metals and the CC.W¹CC exhibits considerably small splitting,where the MO contribution is increased from the auxiliary NO ligand,leading to more MO delocalization and narrower energy splitting.?2?The two 14e MCCs,Ir CC and W²CCs,also exhibit odd-even-property splittings and alternations.Ir CC owns similar STES and HOMO-LUMO gap to Ru CC and Mn CC,while W²CC shows similar but more squeezed STES and HOMO-LUMO gap to W¹CC.Ir CC having a stable 14e configuration gives FMOs similar to Ru CC and Mn CC.In contrast,W²CC owning an unfavorable 14e configuration presents FMOs with one less node than MCCs.The MO contribution from ancillary ligands is also noticeable in W²CC.Apparently,W²CC has different electronic structures than other MCCs.?3?The uncapped linear carbon chain?UCC?exhibits similar but larger splittings and alternations of the STES and HOMO-LUMO energy gaps to MCCs.The FMO shapes in UCC and CC part in MCC are almost identical.The FMO of the MCC indeed results from the overlap of the FMO of the CC and d orbitals of the transition metals.In this case the MO delocalization is increased and the energy splitting narrowed.All the results show that the energy splitting and alternation is a general feature of the MCCs,which is determined substantially by the carbon chain.More attention should be paid to the odd MCCs which own different electronic structures and properties compared to the even.The electronic structure,energy gap and other properties can be tuned by changing the transition metal and ancillary ligands.These observations are helpful for the design of cumulene materials exhibiting tunable electronic and optical properties.