(i) Hydrogen Bonding Studies - The nature of hydrogen bonding in solutions of alcohols, ROH, in various solvents has been studied using nuclear resonance techniques. Data obtained from dilution-shift curves for the OH protons in alcohols have been combined with information derived from infra-red investigations of the OH stretching regions in these compounds. The information obtained has been interpreted in terms of three effects, (i) The electronic effects of the group R. (ii) The steric effects of the group R. (iii) Effects due to other forms of molecular association. On this basis, deductions have been made concerning the degree and type of association in these compounds. The relative hydrogen bonding strengths have been predicted in some instances. The importance of steric inhibition of hydrogen bonding by bulky substituent groups has also been demonstrated. Some dilution-shift studies of acrylic acid in various solvents have been carried out. Studies of the effect of temperature on the OH proton resonances of alcohols have led, amongst other things, to a value for the average hydrogen bond energy in a methanol / carbon tetrachloride solution. The temperature-shift curves for a variety of ortho-substituted phenols have also been obtained and discussed in the light of existing infra-red spectral evidence concerning the nature of hydrogen bonding in these substances. The relation between the association shifts and the integrated absorption intensities of alcohols has been discussed. A correlation between these two quantities was found for alcohols of a similar type. (ii) Studies of Unstable Positive Ions. (a) Triphenyl Carbonium Ions. The NMR spectra of a variety of substituted triphenyl carbonium ions in a trifluoroacetic acid trifluoroacetic anhydride solvent have been obtained at 40 Mc and 60 Mc. No unequivocal evidence as to the structures of these compounds has been obtained, i.e. no distinction between 'symmetrical propeller’ and assymetric forms was possible, due to the presence of exchange effects. The data have given information about the changes in electron density in the aromatic rings due to the various substituent groups. Partial assignments of the aromatic proton spectra have been given. The importance of hyperconjugative electron release by aliphatic substituents is indicated. Some preliminary investigations of the protonated form of 1,1-Di-p-anisylethylene have also been carried out. (b) The l⁺ Ion. The NMR spectra of solutions of iodine in oleum have been investigated to try and shed light on the possibility of the existence of the l⁺ ion in such systems. The measured broadenings and shifts of the oleum proton resonances at various iodine concentrations have been interpreted in terms of the presence of this species, which should be paramagnetic. A value for the magnetic moment of this ion has been obtained. Other evidence for the existence of the l⁺ ion has been fully discussed. / Science, Faculty of / Chemistry, Department of / Graduate

The water molecules in inorganic hydrate salts adopt different geometries and are involved in different hydrogen bond interactions. In this work, magic-angle spinning (MAS) and static 17O solid-state NMR experiments are performed to characterize the 17O electric field gradient (EFG) and chemical shift (CS) tensors of the water molecules in a series of inorganic salt hydrates which include: oxalic acid hydrate, barium chlorate hydrate, sodium perchlorate hydrate, lithium sulphate hydrate, and potassium oxalate hydrate, which were all enriched with 17O water. Data were acquired at magnetic field strengths of 9.4, 11.75, and 21.1 T. Gauge-including projector-augmented-wave density functional theory (GIPAW DFT) calculations are performed on barium chlorate hydrate and oxalic acid hydrate where structural changes including the Ow-H•••O distance, H-O-H angle, and O-H distance are employed to understand their impact on the NMR parameters. Furthermore, simplified molecular models consisting of a metal cation and a water molecule were built to establish the effect the M-Ow distance has on the parameters. The computational studies are then used to understand the experimental results. The 17O quadrupolar coupling constant ranged from 6.75 MHz in K2C2O4•H2O to 7.39 MHz in NaClO4•H2O while the asymmetry parameter ranged from 0.75 in NaClO4•H2O to 1.0 in K2C2O4•H2O and the isotropic chemical shift ranged from -15.0 ppm in NaClO4•H2O to 19.6 ppm in BaClO3•H2O. The computational results revealed the trends for each parameter, where there is an increasing trend for quadrupolar coupling constant and span as a function of increasing hydrogen bond distance, decreasing trend for the three chemical shift tensors as a function of increasing M-Ow distance and unclear trends for asymmetry parameter and skew due to competing electronic factors. Overall, this study provides benchmark 17O NMR data for water molecules in crystalline hydrates, including the first measurement of 17O chemical shift anisotropy for such materials.

Repellents play a key role in preventing mosquito-borne diseases such as malaria by reducing human-vector contact. The general mechanism of action relies on providing a repelling vapour around the applied area on the skin. Thus, the proper evaporation rate and consistency of the composition of the released vapour are factors determining the performance of repellent formulations. The formulation should evaporate fast enough to provide a sufficient level of repellence during its life time. However, if evaporation proceeds too fast, then it will be depleted rapidly so that activity is lost within a short period of time, which makes the repellent inefficient. Several controlled-release approaches have been developed to improve both the protection time and level. However, these techniques have inherent drawbacks from the industrial point of view. Moreover, these techniques mostly focus only on reducing the release rate, while the consistency of the vapour composition has not been addressed. In the present study, a novel approach towards controlling the evaporation behaviour of repellents is proposed. It is based on engineering the molecular interactions in order to design negative pseudo-azeotrope formulations. Negative pseudo-azeotrope mixtures are less volatile than the pure parent components and they do not undergo separation during evaporation. The feasibility of the idea was investigated by studying the molecular structure of generally available repellents. Among known molecular interactions, hydrogen bonding has the most likely impact on the formation of azeotropes and in particular pseudo-azeotropes. Thus, established repellents were classified based on their chemical structures and their capability to take part in hydrogen bonding. Next, a simple spectroscopic method for anticipating pseudoazeotropes formation was developed. Binary compositions of nonanoic acid and ethyl butylacetylaminopropionate (IR3535) showed a potential for forming pseudo-azeotrope mixtures. Hence R3535 and nonanoic acid were selected as model compounds to test the hypothesis. An experimental technique to confirm pseudo-azeotrope formation and to locate the composition of the probable pseudo-azeotrope point was required. To this end, an oven test was designed. The temporal mass loss, under an isothermal program, of a series of evaporating mixtures was measured. Simultaneously, the Fourier transform infrared (FTIR) spectra of the liquid remaining was recorded. Inverse analysis techniques were used to determine the composition of remaining liquid mixtures from the recorded FTIR spectra. The oven tests revealed that, as vaporisation progressed, the composition of the liquid remaining and the emitted vapour converged to a fixed IR3535 content of ca. 75 mol%. Mixtures close to this composition also featured the lowest volatility. Oven test also showed that the composition of the liquid mixtures diverged from the fixed IR3535 content of ca. 10 mol%. Mixtures close to this composition featured the highest volatility. These observations showed that IR3535 and nonanoic acid forms two pseudo-azeotrope compositions, i.e. a negative pseudo-azeotrope at an IR3535 content of ca. 75 mol%, and a positive pseudo-azeotrope at IR3535 content of ca. 10 mol%. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were applied to check these results. TGA confirmed that the negative pseudo-azeotrope mixture is less volatile while the positive pseudo-azeotrope is more volatile than the parent compounds. The DSC results revealed that in comparison with the pure compounds, negative pseudo-azeotrope had a lower boiling point onset while the positive pseudo-azeotrope had a higher boiling point. Although negative pseudo-azeotrope repellent formulations have the desired lower constant release rate, their repellent activity needed to be tested. This is due to the fact that mixing the ingredients to formulate a negative pseudo-azeotrope results in interactions among the components. As a consequence, the inherent repellence effect of the compounds might have been impaired in the mixture. The modified arm-in-cage test was used to test the repellence of the controlled-release repellent formulation i.e. the negative pseudo-azeotrope of the IR3535 + nonanoic acid system. Results showed that the mixture featured improved performance with respect to both repellence efficacy and persistence. Moreover, the negative pseudo-azeotrope also exhibited a knock down effect, even resulting in mortality of most of the test mosquitoes. The presence of two pseudo-azeotrope points at different composition in the IR3535 + nonanoic acid system is a rare occurrence, analogous to double azeotropy. Thus, molecular simulation techniques were used to explore the nature of system and the interactions responsible for this unique behaviour. Gibbs-Monte Carlo simulation results suggest that variations in the sizes of the molecular clusters present in the liquid at various compositions might be responsible. They revealed that IR3535 and nonanoic acid in neat form are both highly structured liquids. The break-down in the structure of IR3535 at high concentrations of the acid may be the origin of increased evaporation rate and formation of the positive pseudo-azeotrope. On the other hand, negative pseudo-azeotrope may be resulted from formation of bulkier clusters at the ration of 3:1 (IR3535: nonanoic acid). / Thesis (PhD (Chemical Technology))--University of Pretoria, 2016. / English / PhD (Chemical Technology) / Unrestricted

Hartree-Fock, Moller-Plesset, and density functional theory calculations have been carried out using 6-31+G(d), 6-31+G(d,p) and 6-31++G(d,p) basis sets to study the properties of low-barrier or short-strong hydrogen bonds (SSHB) and their potential role in enzyme-catalyzed reactions that involve proton abstraction from a weak carbon-acid by a weak base. Formic acid/formate anion, enol/enolate and other complexes have been chosen to simulate a SSHB system. These complexes have been calculated to form very short, very short hydrogen bonds with a very low barrier for proton transfer from the donor to the acceptor. Two important environmental factors including small amount of solvent molecules that could possibly exist at the active site of an enzyme and the polarity around the active site were simulated to study their energetic and geometrical influences to a SSHB. It was found that microsolvation that improves the matching of pK as of the hydrogen bond donor and acceptor involved in the SSHB will always increase the interaction of the hydrogen bond; microsolvation that disrupts the matching of pKas, on the other hand, will lead to a weaker SSHB. Polarity surrounding the SSHB, simulated by SCRF-SCIPCM model, can significantly reduce the strength and stability of a SSHB. The residual strength of a SSHB is about 10--11 kcal/mol that is still significantly stable compared with a traditional weak hydrogen bond that is only about 3--5 kcal/mol in any cases. These results indicate that SSHB can exist under polar environment. Possible reaction intermediates and transition states for the reaction catalyzed by ketosteroid isomerase were simulated to study the stabilizing effect of a SSHB on intermediates and transition states. It was found that at least one SSHB is formed in each of the simulated intermediate-catalyst complexes, strongly supporting the LBHB mechanism proposed by Cleland and Kreevoy. Computational results on the activation energy for catalyzed and uncatalyzed model reactions shows that strong hydrogen bonding between catalyst and the substrate at the transition state can significantly reduce the activation energy. This implies that LBHBs are possibly playing a crucial role in enzyme catalysis by supplying significant stabilizing energy to the reaction transition state.

Proteins are made of vast chains of amino acids that twist and fold into intricate designs. These structures are held in place by networks of noncovalent interactions. One of these, the hydrogen bond, forms bridges between adjacent pieces of the protein chain and is one of the most important contributors to the shape and stability of proteins. Hydrogen bonds come in all shapes and sizes and a full understanding of these not only aids in our understanding of proteins in general but can bridge the gap to finding cures to many protein-related diseases, such as sickle-cell anemia. The primary aim of this thesis is to discover if a specific type of hydrogen bond, the N-H···N bond, occurs within proteins and if so, if it contributes to the structure and stability of proteins.

Room temperature ionic liquids (ILs) are a unique class of solvents which are characterized by non-volatility, non-flammability, electrochemical stability and high ionic conductivity. These properties are highly desirable for ion-conducting electrolytes, and much work has focused on realizing their application in practical devices. In addition, hydrophilic and ionophilic polymers are generally miscible with ILs. The miscibility of ILs with ion-coordinating polymers makes ILs effective plasticizers for gel polymer electrolytes. Due to their unique properties, ILs present a means to realize the next generation of energy storage technology. In this dissertation, the fundamental interactions between poly(ethylene oxide) (PEO) and a variety of room temperature ILs were investigated. ILs with acidic protons were demonstrated to form a stronger interaction with PEO than ILs without such protons, suggesting that hydrogen bonding plays a dominant role for PEO miscibility with ILs. The hydrogen bonding interaction is selective for the PEO block of a PEO-b-PPO-b-PEO block copolymer (BCP). Therefore, blending these copolymers with the strongly interacting IL 1-butyl-3-methylimidazolium hexafluorophosphate ([BMI][PF6]) induced microphase separation into a well-ordered structure, whereas the neat copolymer is phase mixed. At sufficient quantities, the interaction between [BMI][PF6] and PEO suppresses PEO crystallinity entirely. In addition, the induced microphase separation may prove beneficial for ion conduction. Therefore, microphase separated copolymer/IL blends were investigated as potential gel polymer electrolytes. Cross-linkable block copolymers which microphase separate when blended with [BMI][PF6] were synthesized by modifying PPO-b-PEO-b-PPO copolymers with methacrylate end-groups. Cross-linking these copolymers while swollen with an IL generates ion gels with high ionic conductivities. The copolymer/IL blends vary from a well-ordered, strongly microphase separated state to a poorly ordered and weakly microphase separated state, depending upon the molecular weight. Stronger microphase separation results in higher mechanical strength upon cross-linking. However, this does not greatly affect ion conductivity. Nor is conductivity affected by forming gels from cross-linked PEO homopolymers when compared to BCPs. It was found that BCPs can be beneficial in producing gel electrolytes by allowing sequestration of phase selective cross-linkers away from the conducting block. Cross-linker molecules that are selective for the PPO blocks can be used to increase the mechanical strength of the gels with only a small effect on the conductivity. When cross-linkers that partition to the mixed PEO/IL block are used, the conductivity decreases by nearly a factor of 2. These studies show how ILs interact with PEO and how gel polymer electrolytes can be constructed with the IL [BMI][PF6]. While BCPs cannot directly be used to increase ion conductivity, they do allow for greater mechanical strength without sacrificing conductivity. This suggests many new approaches that may be used to simultaneously achieve high ionic conductivity and mechanical strength in solid and gel polymer electrolytes.

No description available.

Non-covalent interactions including nucleobase hydrogen bonding and ionic aggregation were studied in block and end-functional polymers synthesized via living polymerization techniques such as nitroxide mediated polymerization and anionic polymerization. The influence of non-covalent association on the structure/property relationships of these materials were studied in terms of physical properties (tensile, DMA, rheology) as well as morphological studies (AFM, SAXS). Hydrogen bonding, a dynamic interaction with intermediate enthalpies (10-40 kJ/mol) was introduced through complementary heterocyclic DNA nucleobases such as adenine, thymine and uracil. Hydrogen bonding uracil end-functionalized polystyrenes and poly(alkyl acrylate)s were synthesized via nitroxide mediated polymerization from novel uracil-functionalized alkoxyamine unimolecular initiators. Terminal functionalization of poly(alkyl acrylate)s with hydrogen bonding groups increased the melt viscosity, and as expected, the viscosity approached that of nonfunctional analogs as temperature increased. A novel difunctional alkoxyamine, DEPN2, was synthesized and utilized as an efficient initiator in nitroxide-mediated controlled radical polymerization of triblock copolymers. Complementary hydrogen bonding triblock copolymers containing adenine (A) and thymine (T) nucleobase-functionalized outer blocks were synthesized. Hydrogen bonding interactions were observed for blends of the complementary nucleobase-functionalized block copolymers in terms of increased specific viscosity as well as higher scaling exponents for viscosity with solution concentration. Multiple hydrogen bonding interactions were utilized to attach nucleobase-functional quaternary phosphonium ionic guests to complementary adenine-functionalized triblock copolymers. Ionic interactions, which possess stronger interaction energies than hydrogen bonds (~150 kJ/mol) were studied in the context of sulfonated poly(styrene-b-ethylene-co-propylene-b-styrene) block copolymers. Strong ionic interactions resulted in the development of a microphase separated physical network and greater extents for the rubbery plateau in DMA analysis compared to sulfonic acid groups, which exhibit weak hydrogen bonnding interactions. In contrast to the physical networks consisting of sulfonated or hydrogen bonding block copolymers, covalent networks were synthesized using carbon-Michael addition chemistry of acetoacetate functionalized telechelic oligomers to diacrylate Michael acceptors. The thermomechanical properties of the networks based on poly(propylene glycol) oligomers were analyzed with respect to the molecular weight between crosslink points (Mc) and the critical molecular weight for entanglement (Me). / Ph. D.

Current research efforts have focused on the synthesis of novel, segmented, cross-linked networks and thermoplastics for emerging technologies. Tailoring macromolecular structures for improved mechanical performance can be accomplished through a variety of synthetic strategies using step-growth polymerization. The synthesis and characterization of novel Michael addition networks, ionene families, and ion-containing polyurethanes are described, with the underlying theme of fundamentally investigating the structure-property relationships of novel, segmented macromolecular architectures. In addition, it was discovered that both covalent and electrostatic crosslinking play an important role in the mechanical properties of all types of polymers described herein. Novel cross-linked networks were synthesized using quantitative base-catalyzed Michael chemistry with acetoacetate and acrylate functionalities. These novel synthetic strategies offer unique thermo-mechanical performance due to the formation of a multiphase morphology. In order to fundamentally elucidate the factors that influence the kinetics of the Michael addition reaction a detailed analyses of model compounds were conducted in the presence of an in-situ IR spectrometer to optimize reaction conditions using statistical design of experiments. Networks were then prepared based on these optimized conditions. The mechanical performance was evaluated as a function of molecular weight between crosslink points. Furthermore, the incorporation of hydrogen bonding within the monomer structure enhanced mechanical performance. The changes in morphological, thermal, and mechanical properties evaluated using dynamic mechanical analysis (DMA) and tensile behavior are described. In addition, the use of preformed urethane segments provides a safer method for incorporating hydrogen bonding functional groups into macromolecules. In order to compare the thermomechanical and morphological properties of ion-containing polyurethanes to non-charged polyurethanes, poly(tetramethylene oxide)-based polyurethanes containing either a novel phosphonium diol or 1,4-butanediol chain extenders were prepared using a prepolymer method. The novel phosphonium polyurethane was more crystalline, and it was presumed that hydrogen bonding in the non-charged polyurethane restricted polymer mobility, and reduced PTMO crystallinity, and hydrogen bonding interactions were significantly reduced due to the presence of phosphonium cations. These results correlated well with mechanical property analysis. The phase separation and ionic aggregation were demonstrated via wide-angle X-ray scattering, small-angle X-ray scattering, scanning transmission electron microscopy, and energy-dispersive X-ray spectroscopy during STEM imaging, as described herein. In addition, a novel polyurethane containing imidazolium cations in the hard segment was synthesized and behaved very similarly to the phosphonium cation-containing polyurethane. Ammonium ionenes, which contain quaternary nitrogen in the macromolecular repeating unit, have many potential uses in biomedical applications. They offer interesting coulombic properties, and the charge density is easily controlled through synthetic design. This property makes ionenes ideal polyelectrolyte models to investigate the influence of ionic aggregation on many physical properties. Ammonium ionenes were prepared via the Menshutkin reaction from 1,12-dibromododecane and 1,12-bis(N,N-dimethylamino)dodecane. The absolute molecular weights were determined for the first time using an on-line multi-angle laser light scattering (MALLS) in aqueous size exclusion chromatography (SEC). Tensile testing and DMA were used to establish structure-property relationships between molecular weight and mechanical properties for a series of 12,12-ammonium ionenes. Furthermore, degradation studies in the presence of base support the possibility for water-soluble coatings with excellent mechanical durability that are amenable to triggered depolymerization. A novel synthetic strategy was utilized to prepare chain extended 12,12-ammonium ionenes containing cinnamate functional groups. In the presence of UV light, the polymers chain extended, and the resulting ionenes possessed enhanced thermomechanical properties and increased molecular weight. In addition, the novel synthesis of imidazolium ionenes was demonstrated, and the charge density was tuned for appropriate applications using either low molecular weight segments or oligomeric precursors. The change in charge density had a profound role in imidazolium ionene thermal and mechanical behavior. / Ph. D.

In an effort to synthesize macromolecules comprising both covalent and non-covalent bonding to tune ultimate physical properties, a variety of methodologies and functionalization strategies were employed. First, protected functional initiation, namely 3-[(N-benzyl-N-methyl)amino]-1-propyllithium and 3-(t-butyldimethylsilyloxy)-1-propyllithium, in living anionic polymerization of isoprene was used to yield well-defined chain end functional macromolecules. Using both initiating systems, polymers with good molar mass control and narrow molar mass distributions were obtained and well-defined chain end functionality was observed. There was no observed effect on the polymer microstructure from the polar functionality in the initiator, with ~92% 1,4- and 8% 3,4-enchainment observed in each case. Further investigation of the 3-[(N-benzyl-N-methyl)amino]-1-propyllithium initiated polyisoprenes proved that facile deprotection was not possible and residual catalyst was not removable from the polymer. However, polymers initiated with 3-(t-butyldimethylsilyloxy)-1-propyllithium were quantitatively hydrogenated and deprotected under relatively mild conditions to yield hydroxyl functional macromolecules in several architectures, including linear and star-shaped. Excellent conversion from arm polymer to star polymer was observed and well-defined macromolecules were obtained. Subsequently, a series of non-functional, hydroxyl functional, and 2-ureido-4[1H]-pyrimidone (UPy) chain end functional linear and star-shaped poly(ethylene-co-propylene)s were synthesized and characterized. The melt phase properties were investigated using melt rheology and the effect of macromolecular topology and multiple hydrogen bond functionality was investigated. Linear UPy functional poly(ethylene-co-propylene)s exhibited increased viscosity and shear thinning onset at lower frequencies than non-functional polymers of similar molar mass due to interaction of the multiple hydrogen bonding groups. Star-shaped UPy functional poly(ethylene-co-propylene)s showed inhibition to terminal flow and the absence of a zero shear viscosity in melt rheological characterization, indicative of a network like structure imparted from the multiple hydrogen bonding interactions. In addition, the living anionic polymerization of D3 was controlled using the functionalized initiators3-[(N-benzyl-N-methyl)amino]-1-propyllithium and 3-(t-butyldimethylsilyloxy)-1-propyllithium. Good molar mass control and narrow molar mass distributions were observed. In contrast to the polyisoprene homopolymers, facile deprotection of the 3-(t-butyldimethylsilyloxy)-1-propyllithium was not possible due to the acid sensitivity of the poly(dimethylsiloxane) backbone. However, facile deprotection of the protected secondary amine was achieved through hydrogenolysis and well-defined terminal amine functionalized poly(dimethylsiloxane) was synthesized, which are then amenable to further functionalization reactions. In contrast to the well-defined polymers synthesized using living anionic polymerization, free radical polymerizations was used to synthesize free radical copolymers with broader polydispersities and pendant UPy groups. These copolymers were compared with a simple dimeric hydrogen bonding carboxylic acid containing copolymer. Melt rheological characterization revealed that, at similar concentrations, the effect of the UPy group was much greater than the carboxylic acid, and broadened plateau moduli and increased viscosity for the UPy containing polymers were observed, while the acid containing polymer exhibited similar results to a non-functional control. The dynamic viscosity was observed to increase systematically with increasing UPyMA incorporation and the quadruple hydrogen bonding interactions were observed to dissociate between ~80-150 °C. / Ph. D.