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Title: Characterising molecular self-assembly using high-resolution 1H solid-state NMR spectroscopy
Author: Robertson, Aiden James
ISNI:       0000 0004 6062 0519
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 2016
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Supramolecular chemistry, the domain of chemistry ‘beyond the molecule’, is finding increasing application in a diverse range of scientific fields. A key concept in this field, termed molecular self-assembly, has important applications ranging from nanotechnology to medicine, and refers to the intermolecular assembly of individual molecules via non-covalent forces, most importantly hydrogen bonding and π-π stacking interactions. Specifically, this thesis considers the self-assembling arrangements of synthetic pyrimidine-based heterocyclic systems, which present guanine/cytosine DDA/AAD hydrogen bonding motifs. Characterisation of such systems is well documented in the liquid phase, but there exists a general paucity of solid-state analytical data for such materials. Owing to the inherent difficulty in crystallising these systems, typically due to disordered alkyl and aryl sidechains, high-resolution 1H MAS NMR is uniquely placed to elucidate hydrogen bonding arrangements in these systems, given the sensitivity of the 1H chemical shift to its local atomic environment. For cases where single-crystal X-ray structures can be obtained, a so-called NMR crystallographic approach can be used to complement the existing single-crystal X-ray data. By comparison of GIPAW calculated 1H chemical shielding parameters to experimentally observed chemical shifts and, more specifically, through comparison of calculated shielding values for the full crystal versus the isolated molecule, an analysis of the strength of non-covalent phenomenon present in a given system can be presented. This thesis applies fast MAS 1H detected 2D NMR methods to a range of structural problems concerning pyrimidine-based synthetic organic molecules. Specifically,1H – 1H DQ/SQ MAS, 14N – 1H HMQC and 1H – 13C techniques, such as the refocused INEPT and well-known CP MAS methods, are used, where applicable, in conjunction with GIPAW calculated NMR parameters to provide a comprehensive study of the solid-state packing arrangements of a series of guanine/cytosine synthetic derivatives which exhibit a diverse range of self-assembling architectures, including helical and stacking trimeric motifs. It is shown that, for crystalline compounds, confirmation of chemical shift assignments via the GIPAW method can be used to infer the structure of related systems for which diffraction data is not available, through comparison of the observed experimental NMR data. In addition, it is demonstrated that, for a series of pyrimidine and pyridopyrimidine intermediates which form non-crystalline powdered solids, high-resolution 1H MAS NMR methods can be applied alone to elucidate likely hydrogen bonding motifs in the absence of crystallographic data, thereby allowing the observer to speculate on likely modes of self-assembly in the solid state. Interestingly, this study involves the relatively novel investigation (by means of solid-state NMR) of aldehyde and, in particular, oxime containing organic molecules, for which (in the case of the latter functional group) no published 1H MAS NMR studies have been presented at the time of writing. Finally, in collaboration with the spectrometer company JEOL, it is shown that a selective saturation pulse can be employed to supress excessive t1 noise in two-dimensional 1 H MAS NMR spectra, at fast MAS frequencies. To demonstrate this effect, the intense methyl resonance of a synthetic nucleoside derivative is suppressed, whilst reduced spin diffusion rates at higher MAS frequencies ensure that the effect on nearby spins is minimised.
Supervisor: Not available Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QC Physics ; QD Chemistry