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- The Ab Initio Calculation of the Chemical Shift Tensor and its Application for the Structure Determination in Ordered and Disordered Phases by Means of Solid-State Nuclear Magnetic Resonance (2007)
- The intention of this work was the application of modeling to condensed solid-state systems for the sake of the structure determination and prediction. Besides structural modelling this included the ab initio calculation of the chemical shift tensor which was taken for the simulation of various solid-state NMR experiments and the comparison to NMR measurements. In this context it was necessary to create a tool which could deal with the analysis of spin systems from arbitrary ab initio calculations. This was accomplished by the development of the Gau2Sim program library. The application of the included functions to a chemical shift tensor yields the isotropic chemical shift and the anisotropic information in its two established conventions, either expressed via the anisotropy and the asymmetry parameter or via the span and the skew. Additionally the tensor orientation as given by the three Euler angles can be extracted. On the base of this tool the combined approach of structural and NMR modeling was applied to the structure determination in a broad range of disordered, semi-ordered and fully crystalline phases. These systems varied from phases comprising of isolated molecules over hydrogen bonded molecular as well as polymeric phases to covalently bonded two-dimensional layered compounds. For the structural modeling in these environments the advantages of force field and semiempirical methods, density functional and perturbation theory calculations were combined with the description of the systems in the cluster or embedded cluster approach as well as with calculations under periodic boundary conditions. Such it was possible to directly validate structure proposals from diffraction measurements or to screen the structure of yet unknown phases on a broad range. In the latter case low level methods like force field or semiempirical calculations prove especially useful and could serve as a source for the extraction of unit cells from molecular clusters for purpose of calculations under periodic boundary conditions thereby interconnecting both techniques. The NMR properties of the phases were investigated in minimal structure models in this process testing all parts of the chemical shift tensor for their applicability for structure elucidation. Its easiest measurable parameter is the isotropic chemical shift which successfully was used for an interconnection between NMR experiments and structure models. It served well for the purpose of structure verification and in the case of extended 2D layered systems the impact of random local distortion on the chemical shift spectra could be predicted. However, in addition to that the anisotropic part of the NMR tensor could be used to distinguish between different molecular conformers and similar hydrogen bonding environments. Especially in polarizable moieties such as pi systems the anisotropy of the chemical shift is large enough to be evaluated. It proved to be sensitive to the local structural arrangement as well as the electronic environment of the spins. In a test case it could even be shown that the sensitivity of the carbonyl 13C tensor in a hydrogen bridge is large enough to predict the C=O...H-N arrangement through the analysis of theoretical calculated NMR hypersurfaces with the anisotropic parameters from NMR measurements. The most difficult information to analyze is the tensor orientation. This was was done by the measurement of 2D rf-driven spin-diffusion spectra for a 13C isotope labeled system. Through the simulation of 2D patterns on the base of theoretical structure models it was then possible to gain insight about the conformational arrangement of molecules in a disordered environment. Even though elaborate this methodological approach yielded highly detailed information about the local as well as intermediate structure of the solid state which was not available with any other method. An advantage of the method is that it can be applied to ordered and disordered phases alike. This work clearly shows the power of the combination of solid-state NMR measurements with theoretical modeling of the structure and the NMR chemical shift tensor. For future projects the presented Gau2Sim tools provide the basis for a routine analysis of theoretical NMR calculations and a subsequent simulation of spectra which can be compared to arbitrary NMR experiments.