Experimental Measurement And Theoretical Calculation Of Shielding Tensors

Chemical Shift Tensors of spin nuclei in solid contain the important information of electronic distribution around nucleus and of molecular structure, and can be generally expressed by a 3×3 matrix. The measurements of chemical shift tensor by Solid-State NMR are necessary for the determination of material structure. In this dissertation, the theory for measurements of chemical shift tensors is described, and various approaches for NMR measurements of the chemical shift tensors are introduced and compared.Several Solid-State Nuclear Magnetic Resonance (NMR) methods, such as 13C CP-MAS, CP-TOSS and 2D-PASS, were used in the present study to measure the isotropic and anisotropic chemical shifts of a new series of cis-dioxo catecholato molybdenum-tungsten complexes. By using CP-TOSS NMR method to suppress the spinning sidebands, 13C isotropic chemical shift of the above complexes was thus obtained. While in the 2D-PASS NMR spectra, spinning sidebands related to chemical shift anisotropy remain and are well-separated according to their orders. The principal components of the 13C chemical shift tensors were then extracted by fitting the experimental intensities of 13C spinning sidebands with Mathematic programs. The experimental results were also discussed.The quantum chemical computation approaches were used to study the molybdenum complexes system as well. Based on the known structure of the molybdenum complex crystal, theoretical 13C shielding tensors were calculated by ab initio GIAO, Hartree-Fock and DFT (Density Functional Theory) approaches provided by Gaussian98 program. Moreover, the theoretical values of 13C chemical shift tensors were compared with the experimental results and the possible computational errors were discussed. The results indicate that the calculated 13C chemical shift tensors of tungsten or molybdenum metallic complexes are in good agreement with NMR experimental values, indicative of the validity of ab initio quantum chemical computation. The experimental and theoretical studies of 13C chemical shift tensors in metallic complexes provide important information for a further study of the structure, property, reaction mechanism of the complexes.