Understanding of the bonding nature of uranyl and various ligands is the key for designing robust sequestering agents for uranium extraction from seawater.In this paper thermodynamic properties related to the complexation reaction of uranyl(VI)in aqueous solution(i.e.existing in the form of UO2(H2O)52+)by several typical ligands(L)including acetate(CH3CO2),bicarbonate(HOCO2),carbonate(CO32),CH3(NH2)CNO(acetamidoximate,AO)and glutarimidedioximate(denoted as GDO2)have been investigated by using relativistic density functional theory(DFT).The geometries,vibrational frequencies,natural net charges,and bond orders of the formed uranyl-L complexes in aqueous solution are studied.Based on the DFT analysis we show that the binding interaction between uranyl and amidoximate ligand is the strongest among the selected complexes.The thermodynamics of the complexation reaction are examined,and the calculated results show that complexation of uranyl with amidoximate ligands is most preferred thermodynamically.Besides,reaction paths of the substitution complexation of solvated uranyl by acetate and AO have been studied,respectively.We have obtained two minima along the reaction path of solvated uranyl with acetate,the monodentate-acetate complex and the bidentate-acetate one,while only one minimum involving monodentate-AO complex has been located for AO ligand.Comparing the energy barriers of the two reaction paths,we find that complexation of uranyl with AO is more difficult in kinetics,though it is more preferable in thermodynamics.These results show that theoretical studies can help to select efficient ligands with fine-tuned thermodynamic and kinetic properties for binding uranyl in seawater.
Despite its four valence electrons,carbon can at most form triple bond in ordinary organic complexes.Quadruple bonds for carbon had been considered as impossible for a long time.Recently we showed that quadruple bonding is viable in a triatomic uranium carbide oxide molecule CUO,where the terminal C is quadruply bonded with U via its nearly unhybridized 2s-and2p-orbitals.Here we extend this new concept to a series of diatomic molecules consisting of tetravalent p-,d-,and f-elements and terminal carbide.Investigation has been focused on a series of CM-type molecules with possible quadruply-bonded carbon(QBC),CB?,CTi,CZr,CHf,CV+,CNb+,CTa+,and isoelectronic species of CUO.We have performed natural bond orbital(NBO),natural resonance theory(NRT),and atom-in-molecule(AIM)analyses at both density functional theory(DFT)and ab initio CASSCF levels to provide evidence for the feasibility of carbon quadruple bond in these systems.Our calculation results show that the C?M bond orders in these QBC species are comparable to that in CUO,indicating terminal carbides can have novel quadruple bonding when appropriate orbitals are available in the adjacent atoms.
