The Reaction Of Ce Atom And Its Oxide Molecules With N₂ And NH₃ And The Bonding Characteristics

As the most abundant gas on earth,dinitrogen is considered as an economical nitrogen-feedstock to produce ammonia,nitrate or nitrite salts.Ammonia is one kind of the traditional industrial chemicals,which has been widely used in fabrication of fertilizers,nitric acid and ammonium salts.Over the last hundred years,both dinitrogen and ammonia play critical roles in the traditional fundamental chemical industries,such as agriculture,petroleum engineering,and organic engineering and so on.Nevertheless,the inert nature of N=N triple bond leads the functionalization of dinitrogen to be proceeded under high temperature or high-pressure conditions.In addition,the N-H bond activation in NH3,which related to the aminimide of alkene,is known as one of the most puzzled probles in modern catalysis fields.As a result,challenges are directed toward developing economical and effective chemical process to activate dinitrogen and ammonia under mild conditions.Herein,a combined matrix-isolation spectroscopic and density functional theoretical investigation has been carried out on the reactions of Ce atoms,as well as its oxide molecules,with dinitrogen and ammonia.In experiments,"CeO + N2","OCeO + N2" and "Ce + NH3"reaction systems were studied,respectively,in solid argon or neon matrices by changing experimental conditions,including isotopic substitution,annealing,and irradiation.In theoretical investigations,the geometries,vibrational frequencies,and electronic structures of intermediates,transition states and product molecules were calculated using DFT methods(such as B3LYP,BP,and PBE)embedded in Gaussian 09 and ADF 2016 programs.The bonding nature of cerium in certain molecules was analysed by methods of natural bonding orbital,electron localized functional,and atoms in molecules.The conclusions are as followings:1)A group of stable end-on bonded OCeO(NN)1-6 complexes with one to at most six dinitrogen ligands were produced in the reactions of OCeO molecules with dinitrogen ligands.The central OCeO is bent in OCeO(NN)1-4 complexes while it is linear in OCeO(NN)5,6 complexes.Besides,both experimental and theoretical results indicate that dinitrogen ligands are just weakly activated by OCeO.2)Qualitative orbital composition analyses demonstrate that,in linear central OCeO,Ce-4f contributions are obviously increased in the HOMO,penultimate and antepenultimate occupied molecular orbitals.This provides more effective ? overlap between Ce-4f and O-2p orbitals,which may contribute to the stabilization of the linear central OCeO in OCeO(NN)5,6 complexes.3)The CeO(?2-NN)complex is observed in the reactions of CeO molecules with dinitrogen ligands.The N-N stretching band of CeO(?2-NN)complex at 1819.9 cm-1 is about 500 cm-1 lower than that of free dinitrogen molecules(2327.0 cm-1),indicating an obvious and strong activation of dinitrogen,DFT calculations predict the N-N bond length of CeO(?2-NN)complex(1.173 A)is of 0.1 A shorter than that of the free dinitrogen.Bonding analysis suggest that the N=N triple bond is weakened by the ?*back-donation from occupied Ce-5d orbitals to N2-LUMO,which may contribute to the activation of dinitrogen in CeO(?2-NN)complex.4)In the reactions of Ce atoms with ammonia,the spontaneous formation of CeNH3 intermediate on annealing is the initial step of ammonia activation.After overcoming the transition state TS1,the CeNH3 intermediate rearranges to generate the inserted HCeNH2 intermediate on ?>500 nm irradiation.Then,3HCeNH2 intermediate isomerizes into H2CeNH intermediate through transition state TS2 by a-H transfer.The H2CeNH intermediate finally decompose to yield HCeN + H2 upon ?>3 50 nm and ?>220 nm photolysis.5)The periodic trend and differences for reactions and bonding of the group IV M(M =Ti,Zr,Hf,Ce,Th)atoms with NH3 are discussed based on the present and previous works.Among the MNH3 complexes,the symmetric deformation mode of ammonia in CeNH3 complex was observed at 1098.2 cm-1,showing surprisingly a 55 cm-1 decrease than that of the others.This is due to the repulsion between the ammonia lone-pair electrons and that in Ce-4f orbitals.Besides,the Ce-N stretching band of HCeN molecule at 937.7 cm-1 shows a 94 cm-1 higher shift relative to the diatomic CeN molecule,indicating much stronger Ce=N bonding in HCeN.In contrast,the M=N triple bond in HMN(M = Ti,Zr,Hf,Th)molecule is weaker than that in MN molecule.Qualitative orbital interaction and orbital composition analysis suggest that the addition of H ligand to the Ce center will activate the 4f valence shell and increase the strength of covalent between Ce and N atoms.