The Solvation and Regeneration of Ammonia Borane: a Hydrogen Storage Material

Ammonia borane (AB), NH₃BH₃, is considered a promising medium for the storage of hydrogen gas in the solid state, capable of releasing ≤ 16 wt% H₂ at temperatures near 150 ºC. The solvation structures of AB in three solvents: tetrahydrofuran (THF), liquid ammonia, and diglyme, have been probed in depth through neutron scattering experiments. The results were analysed through Monte Carlo simulations based on the empirical potential structure refinement (EPSR) method, in which the simulations are refined to the experimental diffraction data. Hydrogen/deuterium isotopic substitution was used to add constraints to the simulations and improve their accuracy. Full 3D pictures of the bonding in each solvent were determined, all of which were found to be rich in hydrogen and dihydrogen bonding interactions. This approach enabled the answering of several questions pertaining to the spatial and orientational structure of hydrogen and dihydrogen bonding in solution. Among others, the reason for the extremely high solubility of AB in liquid ammonia (72.2 wt%, cf. 20.0 wt% in THF) was discovered to be due to the numerous dihydrogen bonds formed between the AB BHs and ammonia hydrogens, enabling bonding to both ends of the AB molecule unlike in the other solvents. A major challenge preventing the widespread adoption of AB as a green fuel is in the regeneration of the dehydrogenated waste. This has typically been attempted through a three-step process of digestion-reduction-ammoniation, though none have succeeded with mild reagents. In this project the addition of a transesterification step after digestion has been attempted. The digestion product B(OEt)₃ was successfully converted into the more easily reducible B(OC₆F₅)₃ through reaction with pentafluorophenol. Reduction was then attempted with hydrazine in liquid ammonia. A discovered incompatability of pentafluorophenol with ammonia prevented this scheme from working, but B(OC₆F₅)₃ remains a promising target for future attempts at mild reduction

NMR studies of cbEFG-like domains from human fibrillin-1

The calcium binding epidermal growth factor-like (cbEGF) 12-13 domain pair from human fibrillin-1 was the focus of studies for this dissertation. Various nuclear magnetic resonance (NMR) spectroscopy techniques were employed to analyse the calcium binding, structural and dynamic properties of this pair, and to assess the effects of a disease-causing mutation. Fibrillin-1 is a mosaic protein composed mainly of 43 cbEGF domains arranged as multiple, tandem repeats, and mutations within fibrillin-1 have been linked to Marfan syndrome (MFS). 66% of MFS-causing mutations identified thus far are localised to cbEGF domains, emphasising that the native properties of these domains are critical to the functional integrity of this protein. The cbEGF 12-13 pair is found within the longest run of cbEGFs in fibrillin-1, and many mutations that cluster in this region are associated with the severe, neonatal form of MFS. It is thought that this region may be important for fibrillin-1 assembly into 10- 12nm connective tissue microfibrils. Calcium binding studies of cbEGF 12-13 demonstrated that cbEGF 13 contains the highest affinity site thus far investigated from human fibrillin-1. Comparison with previous results showed that fibrillin-1 cbEGF calcium binding affinity can be significantly modulated by the type of domain which is linked to its N-terminus, and also highlighted the high affinity of the "neonatal" region. The NMR solution structure of cbEGF 12-13 is a near-linear, rod-like arrangement of two cbEGF domains, with both exhibiting secondary structure characteristic of this domain type. The rod-like arrangement is stabilised by calcium binding by cbEGF 13 and by hydrophobic interdomain packing interactions. This observation supports the hypothesis that all Class I EGF/cbEGF-cbEGF pairs, characterised by a single linker residue, possess this rod-like structure. The structure also exhibits additional packing interactions to those previously observed for cbEGF32- 33 from fibrillin-1, which may explain the higher calcium binding affinity of cbEGF13. A model of cbEGF 11-15, created based on structural data for cbEGF 12-13 and a model of cbEGF32-36, has highlighted a potential protein binding interface, which encompasses all known neonatal MFS mutations, as well as a flexible, unstructured loop region of cbEGF 12. Backbone dynamics data confirmed the extended structure of cbEGF 12-13. These data, combined with previous data for cbEGF32-33, highlighted a potential dynamics signature for Class I cbEGF domain pairs. Comparison of data for these pairs also suggested that, in addition to the role of calcium in stabilising rigidity on the picoto millisecond time-scale, calcium affinity may play a key role in determining the anisotropy of cbEGF pairs. Possible dynamic explanations for the variation in calcium binding affinity of cbEGF domains from human fibrillin-1 were also noted. The Gl 127S mutation located in cbEGF 13 of fibrillin-1 causes a mild variant of MFS. NMR studies of the G1127S cbEGF12-13 mutant pair showed that cbEGF12 may chaperone folding of mutant cbEGF 13, an effect most likely mediated through interdomain packing interactions. These studies have also shown that the effects of this mutation are localised to cbEGF13, suggesting that a "partial" MFS phenotype is the result of altered structural, dynamic and/or calcium binding properties of this domain

Electron impact ionization studies from atoms to complex molecules and clusters

In this work, a series of electron impact ionization studies on targets ranging from simple atoms to complex systems like clusters are summarized. The momenta of the ionization fragments are measured using a recoil-ion and electron momentum spectrometer called ‘Reaction Microscope’ (ReMi). The objectives of the thesis are two-fold. Firstly, two new target beam sources were constructed in order to enable, for the first time, kinematically complete (e,2e) experiments with the ReMi on substances which are solid at room temperature. As a result (e,2e) experiments were conducted on fundamental lithium and on a complex molecule, 1-Methyl,5-Nitroimidazole, which is among the potential radio-sensitizers under study for radiation therapy. The second part of the work is the study of intermolecular Coulombic decay (ICD) in the biologically relevant systems, thiophene dimers and pyridine D2O dimers. ICD initiated after ionization of the inner valence orbital 7a1 is identified in thiophene dimers. A comparison of the obtained kinetic energy release (KER) distribution with ab initio molecular dynamics simulations indicates that a majority of the ionized dimers have a T-shaped conformational structure. In the hydrated pyridine dimer, it was found that ICD is initiated by the inner valence ionization of O 2s−1 orbital of D2O. ICD initiated from the C 2s−1 and N 2s−1 orbitals of pyridine is assumed to be quenched by the open Auger and electron transfer mediated decay (ETMD) channels

Electron impact ionization studies from atoms to complex molecules and clusters

In this work, a series of electron impact ionization studies on targets ranging from simple atoms to complex systems like clusters are summarized. The momenta of the ionization fragments are measured using a recoil-ion and electron momentum spectrometer called ‘Reaction Microscope’ (ReMi). The objectives of the thesis are two-fold. Firstly, two new target beam sources were constructed in order to enable, for the first time, kinematically complete (e,2e) experiments with the ReMi on substances which are solid at room temperature. As a result (e,2e) experiments were conducted on fundamental lithium and on a complex molecule, 1-Methyl,5-Nitroimidazole, which is among the potential radio-sensitizers under study for radiation therapy. The second part of the work is the study of intermolecular Coulombic decay (ICD) in the biologically relevant systems, thiophene dimers and pyridine D2O dimers. ICD initiated after ionization of the inner valence orbital 7a1 is identified in thiophene dimers. A comparison of the obtained kinetic energy release (KER) distribution with ab initio molecular dynamics simulations indicates that a majority of the ionized dimers have a T-shaped conformational structure. In the hydrated pyridine dimer, it was found that ICD is initiated by the inner valence ionization of O 2s−1 orbital of D2O. ICD initiated from the C 2s−1 and N 2s−1 orbitals of pyridine is assumed to be quenched by the open Auger and electron transfer mediated decay (ETMD) channels