Relativistic Density Functional Theory Study Of Electronic Structures And Spectroscopic Properties Of Bis-imido Uranium Complexes

Uranium nitrides have received increasing attention as a candidate of future nuclear fuel. Due to their high radioactivity, toxicity and scarcity, high-accurate electronic structure theory has become one of the most important tools to explore uranium nitride chemistry. The present theoretical studies have provided reliable data for experimental synthesis of uranium nitrides and their further application in the nuclear industry. In this thesis, relativistic density functional theory(DFT) and time-dependent DFT(TD-DFT) have been used to systematically examine a series of bis-imido uranium complexes for their geometrical/electronic structures, Infrared vibrational spectra, electronic absorption spectra, redox properties and formation reactions. The effects of chirality, steric trans/cis isomerization, oxidation states of uranium(VI and V), and number of bis(imido) uranium core were addressed. Interesting chiral enantiomers have been recognized for the inorganic cis-bis(imido) uranum(VI) complexes. These enantiomers show close geometry parameters and similar electronic structures. The high-lying occupied π(U=N) bonding orbitals were calculated, but the σ(U=N) bonding orbital was found to be energetically stable, which contribute to strong electronic transition spectra in the high-energy area. Compared with the chirality, the steric trans- and cis-isomerization significantly changes geometry parameters and electronic structures. Like the U=O bond of their uranyl analogues, the U=N bond of the bis-imido uranium complexes possesses partially triple bonding characteristics. The latter weaker than the former results in easier transformation between trans- and cis-isomers of the bis-imido uranium complexes. This agrees with the results that cis-bis(imido) uranium complexes were experimentally obtained but no cis-uranyl complexes were reported until now. The reduction from oxidation states of VI to V obviously lengthens U=N bonds and reduces their strengths. The electronic structures and reduction potentials of bis-imido uranium complexes can be tuned by varying the axial substituents, the number of equatorial ligands and the number of the bis-imido uranium core. Optimizations show that binuclear bis-imido uranium(V) complexes have energetically stable electron-spin states of ferromagnetic triplet, paramagnetic singlet and anti-ferromagnetic singlet, allowing for their promising application in magnetic materials. Finally, thermodynamic studies of formation and redox reactions reveal that solvation effects play a predominant role in accurate energetic calculations, while the choice of density functional and the addition of spin-orbit coupling effects have slight effect.