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

摘要

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