An Investigation Of Electronic Structures And Chemical Bonding Of Thorium-contained Molecules

The applications of thorium-contained molecules in thorium-based nuclear fuel,thorium-uranium cycle,the reprocessing of spent nuclear fuel,catalysis are related to the basic physical and chemical properties as well as the interactions between thorium and various ligands.Under the background,this dissertation applied anion photoelectron velocity-map imaging?VMI?spectroscopy to the measurement of electron affinities?EA?and fine structures of different thorium-contained molecules,while combined it with the high-level quantum chemical calculations to perform a thorough research on ThO_x?x=1-3?,ThC,ThN,ThF_x?x=1,3?,aiming at investigating the geometric structures,electronic structures,vibrational frequencies,electron affinities and chemical bonding of different thorium-contained molecules.The research results are as follows:?1?Through the accurate treatment of electron correlation and relativistic effects,the CASSCF/CASPT2/SO method can accurately calculate the ground and excited states of ThO and ThO^-and then reliably analyzed the anion photoelectron spectroscopy.?2?For the first time we obtain the electron affinity of some thorium-contained molecules experimentally as:EA?ThO?=0.707±0.020 eV,EA?ThO₂?=1.21±0.05eV,EA?ThC?=1.103±0.005 eV,EA?ThN?=1.577±0.005 eV,EA?ThF₃?=1.176±0.005 eV.?3?Research on chemical bonding demonstrates that,among all the thorium-contained molecules studied in this dissertation,the thorium atom achieves bonding to different ligands through the df hybridized orbitals.The Th-C bond has relatively obvious sdf hybridization in ThC.?4?For ThO_x?x=1-3?molecules:Th-O are all multiple bond;Compared with ThO,the natural population of Th-7s for the latter two decreases significantly,whereas the natural population of Th-6d and Th-5f increase obviously,df hybridization facilitates Th-O bonding.For ThM?M=C,N,O,F?molecules:From Th-C to Th-F,the bond length decreases first and then increases?Th-N is the shortest?,the bond order and covalency decrease in turn;Th-C and Th-N are all multiple bond;the effective bond order of Th-F is less than 1.?5?For EThF₂?E=O,S,Se,Te?molecules:Th-E are all multiple bond;Research on chemical bonding shows that the covalency of Th-E is gradually increasing from O to Te.This study can deepen the understanding of chemical bonding in f-elements and has guiding significance of screening ligands based on the differences of covalency,thereby the separation of minor actinides and lanthanides in spent nuclear fuel can be further realized.