Theoretical Studies On The Structural And Functional Properties Of Actinide Embedded Superatoms

The actinide elements are in the f-block in the periodic table and their ground states are 5f⁰¹⁴6d⁰²7s².They have a number of unfilled electron shells,thus the whole molecular system containing actinide elements will display extremely abundant of electronic structures.Compared with lanthanide elements,the 5f electrons of actinide elements are not localized as strong as the 4f electrons of lanthanide elements,and their energy are very close to the 6d electrons.Which means that 5f electrons are more chemically active than 4f electrons.Although the physicochemical properties of the light actinide elements are not completely dominated by 5f electrons,the 5f electrons are in the valence shell and play an important role in bonding and interaction with other atoms.Special structures can be generated when actinide elements interact with nanoscale confined structures.There are a lot of systems who can confined actinide elements,such as the cage and ring structures composed of metals in the ds region,the cage structures composed of carbon and boron elements containing p electrons,providing plenty samples for both basic and applied research.In this paper,based on the Density Functional Theory?DFT?,the structure properties,spectral properties of a series of actinides embedded superatomic structures were studied.Also,the reliability of DFT in actinide endohedral system was analyzed.For embedded systems,two-dimensional planar structures are of great important.There are only in-plane orbitals interacting strongly,while the out-plane orbital orbitals interact weakly.This makes two-dimensional superatoms have a lot of special properties in terms of tunable optics and magnetism,comparing with three-dimensional superatoms.A series of nearly-planar superatom clusters An@Au₆(An=Ac⁺¹,Th,Pa^-1)were designed,with light actinide atoms embedded in gold rings.The electronic structure analysis showed that these nearly-planar superatoms have a ten-electron?1S²1P⁴1D⁴?closed-shell singlet configuration with only in-plane two-dimensional occupied superatomic molecular orbitals?SAMOs?.Energy decomposition analysis and charge density difference analysis showed the non-negligible covalent character between embedded atom and Au₆ ring.With f electrons are getting closer to the frontier orbitals by the increasing atomic number of actinide element,the disturbance of f electrons to the front orbitals is getting stronger,uncovering U and Np atoms cannot form stable planar structure with Au₆ ring.Compared with the traditional Surface-enhanced Raman scattering metal substrate,the SERS properties involved in actinide embedded superatom may be very novel due to the strong delocalization of 5f and 6d electrons.The deformation density analysis revealed that the electron exchanges mainly take place in the junction of the pyridine-metal complexes,thus our systems are suitable to be used as the SERS metal substrate.The SERS spectroscopic showed that with pyridine absorbed on actinide elements,the Raman signal were enhanced by 10¹0⁴.For pyridine–Th@Au₆ complex,the charge-transfer?CT?states at 677 nm(1D_metal–?₁*_pyridine)can lead to SERS signal enhancement by 10⁴.It is time-consuming by using post Hartree-Fork method to calculate actinide embedded superatoms with complex electronic correlation effect.Therefore,finding a reliable DFT method appears very important.There is obvious violation to the Jacob's ladder by specific DFT methods of carbon based high-Z elements endohedral metallofullerenes?hZ-EMFs?.Whether DFT methods are dependable for actinide embedded superatoms are becoming an urgent issue.Hence,different DFT methods were choose to calculate U@B₄₀,who is the first actinide embedded superatoms based on boron cage structures predicted by calculation.DFT calculations revealed that the conclusion of its electronic structure and geometric structure depends heavily on the choice of functional type.Pure generalized gradient approximations?GGA?give a high symmetrical singlet ground state of D_2d point group,while a slightly tortured C_2v triplet state is more stable when hybrid GGAs are used.The spin density analysis shows that in the pure functional triplet structure,the spin direction of the U atom is the same as that of the B₄₀ cage,and the U atom contributes most of the spin.In the hybrid functional triplet structure,the spin direction is opposite,and the spin density accumulates more obviously on B₄₀0 cage than that on the pure functional triplet structure.The qualitative difference originates in the proportion of exact exchange energy that could exert a tremendous influence on energy splitting between electronic states of U@B₄₀.We hope that great care should be taken when DFT methods are used to investigate hZ-EMFs.In summary,this work studied the electronic structure and SERS spectroscopy applications of actinide embedded superatoms.In addition,the reliability of DFT method in calculating actinide embedded superatoms was also explored.These will provide important support for experimental and theoretical researches of such systems and their derivatives.It is of great significance to design new materials"bottom-up”by embedding high Z elements to change the properties of materials according to different uses.Therefore,a clear understanding of the structural and functional properties of high-Z elements embedded superatoms not only has valuable for basic research,but also shows applications prospect.