Theoretical Studies Of The Slab Structure And Supermolecular Interactions Of ZnAl-LDHs

In this work, the electronic structures of haloid anions intercalatedZnAl-LDHs have been studied by using density functional theory (DFT)based on the moleculer cluster models. Stable structures of the differentintercalated products are obtained through optimizing geometries of moleculercluster models of LDHs. The calculation results of the frontier molecularorbitals, vibrational frequencies and interaction energies reveal thesupermolecular interactions between the host slabs and the guest anions. Inaddition, the structures of Zn(OH)₆^4-, Zn(H₂O)₆²⁺ and AlZn_m(OH)_2m (m=1～6)complexes have been investigated, and the growth process of Zn-Al slab hasalso been analyzed. These works offer the theoretical basis to understand themechanism of the formation of LDHs slab. The main results are as following:(1) Theoretical Study on the Electronic Structures of Zn(OH)₆^4- andZn(H₂O)₆²⁺A series of complexes of Zn(OH)₆^4- and Zn(H₂O)₆²⁺ with differentsymmetries of point groups have been optimized by using density functionaltheory at the B3PW91/Lanl2dz, B3PW91/6-31G(d,p) andB3PW91/6-311++G(d,p) levels. The frontier molecular orbitals and IRspectrums of the complexes are calculated at the B3PW91/6-311++G(d,p)level. The calculated results show that, Zn(OH)₆^4- with symmetry of D_3d pointgroup has the lowest energy among complexes of Zn(OH)₆^4-, and Zn(H₂O)₆²⁺with symmetry of T_h point group has the lowest energy among complexes ofZn(H₂O)₆²⁺. According to the system energies, the system [Zn(OH)₆^4-+6H₂O]is more stable than the system [Zn(H₂O)₆²⁺+6OH^-]. The O-H bands havestrong absorptions at 3816 and 1638 cm^-1, the Zn-O bands have strongabsorptions at 541 and 391 cm^-1, which are close to the experimental data. Itproves that the models are appropriate to study the electronic structures ofZn(OH)₆^4- and Zn(H₂O)₆²⁺.(2) Theoretical Studies on Delamination of ZnAl-LDH-ClThe geometries of delamination of ZnAl-LDH-Cl models are optimized bydensity function theory at the B3PW91/Lanl2dz levels. Several different kindsof stable delamination structure models are obtained and the frontier molecularorbitals, vibrational frequencies and supermolecular interactions between thehost slabs and the guest anions of LDHs are also calculated. The results showthat model which Cl^- located right above the center atom Al has the lowestenergy. The interatomic distances from the interlayer anion to the slab denotethat there exist hydrogen bond interactions between the hosts and guests. The charge analysis for the natural bond orbitals (NBO) shows that the electrostaticinteraction is an important component of supermolecular interactions betweenthe slab and the anion. According to the molecular orbital calculations it canbe seen that the electrons transfer from the highest occupied molecular orbital(HOMO) of halide anion to the lowest unoccupied molecular orbital (LUMO)of slabs in the adsorption process.(3) Theoretical Studies on Supermolecular Interactions of HaloidAnions Intercalated ZnAl-LDHsThe geometries of haloid anions intercalated ZnAl-LDHs models areoptimized by density function theory at the B3PW91/6-31G(d,p) andB3PW91/6-311++G(d,p) levels, respectively. Several different kinds of stablelayered structure models are obtained and the frontier molecular orbitals,vibrational frequencies and supermolecular interactions between the host slabsand the guest anions of LDHs are also calculated. The results show that thebond parameters of the structural models calculated from two kinds of basissets are very close. The interatomic distances from the interlayer anion to theslab denote that there exist hydrogen bond interactions between the hosts andguests. The charge analysis of the natural bond orbitals (NBO) shows that theelectrostatic interaction is an important component of supermolecularinteractions between the slab and the anion. According to the molecular orbitalcalculations it can be seen that the electrons transfer from the highest occupiedmolecular orbital (HOMO) of halide anion to the lowest unoccupied molecularorbital (LUMO) of slabs in the intercalated process. The stability of thesystems decreases in the order F-ZnAl-LDHs＞Cl^--ZnAl-LDHs＞Br^--ZnAl-LDHs.(4) Theoretical study on the Slab growth mechanism of ZnAl-LDHsThe possible models of 2～7 polymers of Zn-Al double hydroxides withsame charges are studied at the B3PW91/6-31G(d,p) level. The results showthat under the condition of same sort and number of atoms the system energiesdecrease with increasing the polymer number, while the isomers with the mostOH groups at three-centered bridging positions have the lowest the energies. Itindicates that the growth of LDHs slab a spontaneous possess.