Theoretical Study On Redox And NLO Properties Of Keggin And Lindqvist POMs

Polyoxometalates (POMs) are a rich and diverse family of metal–oxygen clusters made up of early transitional metals with unique photonic, electronic, and magnetic properties and chemical reactivity that has promised dramatic applications in quite diverse disciplines, including catalysis, medicine, and materials sciences. Following the development of the computer technology, quantum chemistry calculation as a theoretical study method has been introduced to the field of POMs. In early time, the ab initio Hartree–Fock (HF) approximation provided a reasonable starting point for understanding the structural and electronic properties of POMs. However, it is necessary to analyze systematically molecular and electronic structures, redox, catalysis and optics of POMs, consequently these could direct synthesis and material preparation. In the last ten years, a few groups have been especially active and have made important progress in describing and rationalising the electronic and magnetic properties of POMs. But a combination of three factors—the large size of polyoxoanions, the presence of transition metal ions and the high negative charge—produce, in practice, important computational limitations. The ab initio and density-functional theory (DFT) modeling of polyoxoanions is still incipient.In this thesis, quantum chemistry calculations have been carried out to investigate electronic properties, stability, redox, and the second-order nonlinear optical (NLO) properties of a series of Keggin-type POM derivatives. The present work has been focus on the following three aspects:1. Density Functional Theory (DFT) calculations were carried out to investigate the electronic and redox properties ofα-[XW12O40]n- (X=P and Si) and their derivatives [XW11M(H2O)O39]m- (X=P, Si and M= W, MnⅢ, FeⅢ, CoⅡand NiⅡ). It leads to the conclusion that the hydrogen atoms only significantly modify the LUMO of [SiW12(H2O)O39]2-, and the HOMO in [SiW12(H2O)O39]2- has no significant change comparing withα-[SiW12O40]4-. However, the substituted transition metal can not only modify LUMO in [SiW11M(H2O)O39]m-, but also for the highest single occupied molecular orbital (SOMO) in [SiW11M(H2O)O39]m-. The calculations also show that the replacement of hydrogen atoms makes the system reduce more easily. Therefore, the substitution of the transition metal makes the redox reactivity more difficult. In the same central heteroatom systems, the redox reactivity of the systems substituted by the trivalent metal is higher than the systems substituted by bivalent metal. In other words, the charge of the system is an extraordinary factor which affects the redox property. Finally, the substituted transition metal atom prefers to accept the extra electron when the system is reduced.2. Density Functional Theory (DFT) calculations were carried out to investigate the electronic and redox properties of Keggin POMsα-[XW12O40]n- (X=P and Si) and their derivatives. The molecular structures ofα-[XW12O40]n- are almost unaffected by the solvent. While the LUMO energies of the systems decrease with the increasing of the solvent polar. In contrast, the redox energies increase. That means the polar of the solvent have great effect on the system's redox property, and the system's redox is stronger in the polar solvent than in the non-polar solvent. In order to prove this, we calculated the Keggin [PW12(H2O)O39]- and [PW11Co(H2O)O39]5-. From the calculation results, the molecular redox ability increase with the polarity of the solvent.3. The static first hyperpolarizabilities and origin of nonlinear optical (NLO) properties of [(2-methylnaphthyl) imido] hexamolybdates derivatives have been investigated by density functional theory (DFT). The [(2-methylnaphthyl)imido] hexamolybdate has considerable large first hyperpolarizability, 6.780×10-30 esu, and it is larger than that of [(2,6-dimethylphenyl) arylimido] hexamolybdate due to the double aromatic rings in the naphthylimido ligand. The naphthylimido ligand acts as an electron-donor and the polyanion acts as an electron-acceptor. The substituent position on the naphthylimido is a key factor to determine the first hyperpolarizability of (naphthylimido) hexamolybdate derivatives. The derivative, which the iodine atom locates on the para nitrogen on the naphthylimido ligand, has the largestβ0 value among the iodine-substituted derivatives. It suggests that the iodine atom is quasi linear with nitrogen and Mo, which is bonded to the nitrogen atom, could generate a large static electronic field and give the large contribution to NLO response. The introducing of electron-donors significantly enhances the first hyperpolarizabilities of (naphthylimido) hexamolybdates comparing with the electron-acceptors as the electron-donating ability is significantly enhanced when the electron-donor is attached to the naphthylimido segment. The present investigation provides important insight into NLO properties of (arylimido) molybdate derivatives.