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Probing intermolecular interactions and three-dimensional packing of organic molecules by solid-state NMR

By Amy L. Webber

Abstract

Identifying the ordered three-dimensional structures formed by atoms\ud and molecules is essential to understanding the properties of solid-state materials.\ud Solid-state NMR is an extremely sensitive structural probe and offers\ud atomic-level information regarding the three-dimensional packing of molecules\ud and the intermolecular interactions, for example, hydrogen bonding, which\ud control this. Recently, the combination of advanced solid-state NMR experiments\ud and complementary computational techniques have led to the emergence\ud of the field of `NMR crystallography', which shows great potential for the structural\ud determination of systems where traditional diffraction-based methods are\ud not suitable.\ud The work in this thesis uses a combined approach of high-resolution\ud MAS NMR experiments and first-principles (GIPAW) calculations of NMR\ud parameters to provide structural insight into a range of challenging organic\ud systems. In particular, 1H-13C and 1H DQ (double-quantum) CRAMPS (combined\ud rotation and multiple pulse spectroscopy) techniques are employed to\ud identify 1H and 13C NMR chemical shifts and close 1H-1H interatomic proximities.\ud A new 1H DQ-13C SQ (single-quantum) experiment is presented that\ud better allows intra- and intermolecular 1H-1H distances to be identified in the\ud pharmaceutical compound, penicillin and the disaccharide, β-maltose monohydrate,\ud notably enabling, for the first time, the full 1H resonance assignment\ud of the latter. Using a similar methodology, a `spectrum to structure' approach\ud is applied to identify modes of self assembly for guanosine derivatives for which\ud single-crystal diffraction structures could not be obtained. In addition, chemical\ud shift calculations on the full unit cell (348 atoms) of a complex pyrazole\ud allow the complete assignment of experimental 1H, 13C resonances for each of\ud the six independent molecules of the asymmetric unit cell. Finally, hydrogen-bond\ud mediated 2hJ15N17O and 2hJ15N13C couplings across NH...O and N...HC\ud hydrogen bonds are determined experimentally for the first time by the use of\ud heteronuclear spin-echo experiments. The J couplings, which have also been\ud determined using first-principles calculations, are a quantitative measure of\ud hydrogen-bonding strength

Topics: QC
OAI identifier: oai:wrap.warwick.ac.uk:3814

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