Theoretical Study On The Electronic Structures Of [Fe(H₂O)_m(OH)_n]～x And MgFe-LDHs

In this paper, by using the B3PW91 method of density functional theory (DFT), the possible structures of [Fe(H₂O)_m(OH)_n]^x(m+n=4,5,6) have been optimized, the geometric parameter, charge distribution and energy of complexes have been analyzed; from the point of view of multi-metal complexes, the growth process of MgFe-LDHs' monolayer has been explored; based on the building of the host and guest model, the electronic structure and interaction of the host and the guest between MgFe-LDHs' monolayer and CI or NO3 have been investigated; the structural models of MgFe-X-LDHs have been optimized, the geometric parameter, charge distribution and interaction energy of the host and the guest have been analyzed. The main research contents and results are as following:(1) Theoretical study on the electronic structures of [Fe(H₂O)_m(OH)_n]^xThe possible stable structures of [Fe(H₂O)_m(OH)_n]^x(m+n=4,5,6) have been optimized at the B3PW91/6-31G(d, p) level. When m+n=4, there are five kinds of stable structures, [Fe(OH)₄^-+6H₂O+2OH] is the most stable; When m+n=5, there are five kinds of stable structures, [Fe(OH)₅²+6H₂O+OH] is the most stable; When m+n=6, there are three kinds of stable structures, [Fe(OH)₆^3-+6H₂O] is the most stable.(2) Theoretical study on the growth process of MgFe-LDHs' monolayerThe growth process of MgFe-LDHs' monolayer has been studied at the B3PW91/Lanl2dz level, the computational results indicate: from the beginning of MgFe(OH)₈^3-, the layer's structure gradually grow up to Mg₆Fe(OH)+(12)³⁺ along the way: MgFe(OH)₈^3-—Mg₂Fe(OH)₉^2-—Mg₃Fe(OH)₁₀^-—Mg₄Fe(OH)₁₁—Mg₅Fe(OH)₁₂⁺—Mg₆Fe(OH)₁₂³⁺. Then, the layer's structure continues to grow up to Mg₃₀Fe₇(OH)₇₂⁹⁺ centred on Mg₆Fe(OH)₁₂³⁺. During the growth process, the energies of the system decrease gradually and the layer's structure becomes more stable. (3) Theoretical study on supra-molecular interaction between MgFe-LDHs' monolayer and Cl^- or NO₃^-The host and guest models between MgFe-LDHs' monolayer and Cl^- or NO₃^- have been optimized at the B3PW91/6-311++G(d, p) level, the geometric parameter, charge distribution and interaction of the models have been analyzed, the computational results indicate: there are strong supra-molecular interactions between the host layer and a guest anion Cl^- or NO₃^-, supra-molecular interactions are resulted mainly from the hydrogen-bond and the electrostatic interaction, the host-guest interaction energies of Mg₆Fe(OH)₁₂³⁺: Cl and Mg₆Fe(OH)₁₂³⁺:NO₃^- are -1020.7944 and -956.8944 kJ·mol^-1. (4) Theoretical study on the electronic structures of MgFe-X-LDHsThe structural models of MgFe-X-LDHs(X=F^-, Cl^-, Br^-) have been optimized at the B3PW91/6-31 l++G(d, p) level, the geometric parameter, charge distribution and interaction of the models have been analyzed, the computational results indicate: there are strong supra-molecular interactions between the host two layers and a guest anion F^-, Cl^- or Br^-, supra-molecular interactions are resulted from the hydrogen -bond and the electrostatic interaction, during the process of intercalation, the host two layers keep stable and the energies of the systems decrease, the host-guest interaction energies of Mg₆Fe(OH)₁₂³⁺: F-^, Mg₆Fe(OH)₁₂³⁺: Cl^- and Mg₆Fe(OH)₁₂³⁺: Br^- are -400.6513, -340.2648 and -325.8246 kJ·mol^-1, the stability order is as following: MgFe-F-LDHs>MgFe-CI-LDHs> MgFe-Br-LDHs.