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Theoretical Methods and Applications of Computational NMR


Computational NMR has been used to assign the structure of numerous organic compounds, but limitations still exist on the accuracy of 1H and 13C chemical shift prediction when, for example, there are strong interactions present in the NMR solution, large compounds with high degrees of flexibility, or multiple experimental chemical shifts are within the expected deviation for the level of theory. In this work, 1H and 13C chemical shifts have been predicted for the structural assignment and reassignment of several natural and synthetic products and many of the current limitations of computational NMR are addressed. In Chapter 1, an overview of how theoretical chemical shifts are calculated for 1H and 13C NMR is covered with the current limitations and sources of error in the field. Chapter 2 focuses on method development for computational 1H and 13C NMR. In the first part of the chapter, the underlying assumptions of using an implicit solvent and empirical dispersion in the optimization of the compounds used, as well as including conformational flexibility, in the calculations for the proton and carbon isotropic values for scaling factors to convert isotropic values to chemical shifts have been assessed. The second part of Chapter 2 discusses the development of calculating chemical shifts for alkaloids in different levels of acidity for NMR spectra collected in CDCl3. Chapter 3 covers the prediction of 1H and 13C chemical shifts for a variety of synthetic and natural products ranging from diterpenoids to cyclopeptides. Structural reassignments, assignments of chemical shifts difficult to confirm with experimentation, and conformational analysis for a range of synthetic and natural products is presented.

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