DFT Study On The Supermolecular Structure Of ZnFe/NiAl LDHs

The possible geometric structures of Ni( II) aqua and hydroxyl complexes were optimized by means of density functional theory (DFT) at B3PW91/Lanl2DZ and B3PW91/6-31G^** levels, respectively. The geometric parameters, energy, frontier molecular orbital, atomic charge, and vibrational frequencies were analyzed and compared. The growth mechanism and formation rule of the LDHs layer clusters [Zn_nFe(OH)_2n]³⁺å’Œ[Ni_nAl(OH)_2n]³⁺ were studied by the B3PW91/LAN2DZ method of the DFT theory. The single layer model contained one ZnFe-LDH lamella and one anion(Br^-, NO₃^- ) and the double layers model contained two lamellas and one anion(F^-, Cl^-, Br^-) were optimized at B3PW91/LAN2DZ level. The vibration and Natural Bond Orbital (NBO) was studied. The bond parameters, IR spectra, charges population and interaction energies between the host and the guest were analysised. Main results are as follows:1. Density functional theory study on the electronic structures of Ni(II) aqua and hydroxyl complexes:The results show that the Ni-0 bond lengths of the calculated results are in good agreement with those of the experiments. The structure of Ni(H₂O)₆²⁺ is more stable than that of Ni(OH)₆^4- , but the latter is proner to attract cations. The shapes of frontier molecular orbital also show that the highest occupied molecular orbitals (HOMO) of Ni(OH)₆^4- are mainly located on the O atoms of the ligands, which is beneficial for charge transferring between the ligands and the acceptors and bonding with the cation group. The IR spectrums of the Ni complexes consist with the experiment data as well.2. Study on the growth mechanism of LDHs layer clusters [M²⁺_nM³⁺ (OH)_2n]³⁺ (M²⁺=Zn,Ni;M³⁺=Fe,Al;n=2-6):The stability of [Zn_nFe(OH)_2n]³⁺å’Œ[Ni_nAl(OH)_2n]³⁺ depends on the number of bridging position OH. In the most stable structure, all of the Metal ions are nearly on the same plane. It shows that [M²⁺_nM³⁺(OH)_2n]³⁺ (M²⁺=Zn,Ni;M³⁺=Fe,Al;n=2-6) is inclined to grow adopting an layered structure, which is in agreement with the LDHs structure we have found through the experiments. In this process, we use the trimer [M²⁺₂M³⁺(OH)₆]³⁺ as the basic nuclear and M²⁺(OH)₂ as the cell. The lamella grows in a way that ensures the number of bridging position OH is the most and behaves periodicity and circularity. The stabilization energyâ–³E is linear to the number n of the M²⁺ atoms and the calculatied stabilization energy depends on the number of M²⁺ atoms. It suggests that the larger the lamella size is, the structure tends to be more stable as the size of lamella increases.3 Study on the super-mocular interaction between the host layer and guest anion of ZnFe LDHs:A slight strong interaction exists between the single host layer and guest anion of ZnFe LDHs. The interaction order is: NO₃^--, which is consistent with the results of experiments from the literature. An slight strong interaction exists between the double host layers and guest anion of ZnFe LDHs, as well. The interaction order is: F^->Cl^->Br^-. Beacause the ion exchange order is:Br^- - - , which is also in agreement with the experimental data.