High-resolution NMR studies of solid halogenated organic compounds

This thesis is a study of solid halogenated organic compounds by Nuclear Magnetic Resonance Spectroscopy (NMR) in an attempt to extract previously inaccessible information. The first part of the thesis is concerned with three fluorinated steroids, studied by observing (^1)H, (^13)C and (^19)F nuclei. A number of experimental techniques are employed to verify solution-state and solid-state spectral assignments, and spectral anomalies are discussed. Both proton-coupled and proton-decoupled (^19)F solid-state spectra, recorded using specially designed spectrometer hardware, are presented. The huge gain in resolution afforded by the implementation of proton decoupling allows static and MAS spectra to yield previously inaccessible information pertaining to various NMR parameters of the fluorine nuclei. Advantages of (^1)H→(^19)F cross-polarisation experiments over single-pulse experiments are explained and rotational resonance, dipolar dephasing, T(_1), measurement and spin-exchange experiments are presented from which information regarding phenomena such as spin diffusion and polymorphism is gleaned. The second part of the thesis focusses on the topic of residual dipolar coupling, the transfer of quadrupolar effects to spin-1/2 nuclei via dipolar coupling and/or anisotropy m indhect coupling. Unexpected, field-dependent, multiplicities for signals in spectra of spin-1/2 nuclei are observed, which can be used to evaluate certain fundamental NMR parameters including the quadrupolar coupling constant and, m favourable cases, anisotropy in indirect coupling. The phenomenon is comprehensively studied for the (^13)C, (^35,37)Cl and (^13)C, (^79,81)Br spin-pairs in a range of solid halogenated compounds. Coupling to more than one halogen nucleus and long- range (non-bonded) coupling are considered. First-order perturbation, inverse first- order and "exact" theories, that allow the multiplet line positions to be predicted, are introduced and their results are subsequently compared to the experimentally observed the positions. Rapid molecular motion is shown to negate the effects of residual dipolar coupling and the phenomenon is analysed with the aid of NQR measurements

Nuclear quadrupole resonance system for landmine detection in Antioquia

Colombia ranks second in the world by number of victims from landmines; and in Colombia, Antioquia is the most affected department. As most landmines in Antioquia do not have metallic parts, metal detectors became useless, however, in most of those mines, the explosive mixture includes ammonium nitrate, and thus a system able to detect this compound could help to locate the mines. On the other side, nuclear quadrupole resonance (NQR) is a spectroscopic technique that allows the detection of some compounds very specifically. Thus, this work had the purpose of developing a system for remote sensing of ammonium nitrate in landmines by NQR. To achieve this goal, a portable NQR device was constructed as well as a probe, capable of sending radiofrequency pulses at the resonance frequency of ammonium nitrate, and capable of picking up the NQR signal after excitation. The manufactured system was tested against some environmental factors, concluding that the most affecting ones are soil conductivity and landmine shape. After achieving a functional system, a classifier based on spectral descriptors was trained, using data acquired with and without ammonium nitrate. Experimental results showed that the proposed classifier (an ensemble of 20 decision trees) had better performance, in terms of the area under the receiver operating characteristic curve, than the classical solution on the literature (which relies only on signal intensity). A final test validated the performance of the system, which detected four of five buried targets in an area of 2 x 1,6 m, having 3 false alarms.Resumen: Colombia es el segundo país con mayor número de víctimas por minas antipersona (MAP) en el mundo, siendo Antioquia es el departamento más afectado. La mayoría de las MAP en Antioquia contienen nitrato de amonio y la resonancia nuclear en cuadrupolo (NQR) es una técnica espectroscópica que permite detectar compuestos de forma muy específica. Así, este trabajo tiene el propósito desarrollar un sistema de NQR para la detección remota de nitrato de amonio en MAP. Para lograr este objetivo, se construyó un equipo portable de NQR, así como un inductor capaz de enviar pulsos a la frecuencia de resonancia del nitrato de amonio y de detectar la señal de NQR. El sistema construido fue probado en diferentes condiciones ambientales y se encontró que los factores que más lo afectan son la conductividad del suelo y la forma de la mina. Luego de lograr un sistema portable y funcional, se entrenó un clasificador basado en descriptores espectrales usando datos adquiridos con y sin nitrato de amonio. A partir de resultados experimentales se encontró que el clasificador entrenado (un ensamble de 20 árboles de decisión) tiene mejor desempeño, en términos del área bajo la curva de característica operativa del receptor, en comparación con la solución extendida en la literatura (que se basa únicamente en la intensidad de la señal). Una última prueba validó el desempeño del sistema, que fue capaz de detectar 5 muestras de nitrato de amonio ocultas en un área de 2 x 1,6 m, con 3 falsas alarmas.Doctorad

Zero-field Nuclear Magnetic Resonance of Copper Minerals

Industrial magnetic resonance spectrometers are used for on-line, bulk, quantitative analysis of minerals. These analysers are used to rapidly detect the grade of ore which can assist ore-sorting and advanced ore management to overcome declining mine productivity due to the gradual reduction in mined ore grade. Magnetic resonance is well suited to these bulk sensing applications. The technique provides high levels of mineral discrimination as it is highly sensitive to subtle changes in electric, magnetic, and crystallographic environment of copper in target minerals. Being a radio spectroscopy, it also delivers significant advantages in bulk measurement of ore materials due to the penetrating nature of radio excitation. On-line mineralogical characterisation can be achieved without the use of extremely high magnetic fields associated with conventional nuclear magnetic resonance. The two sub-classes of interest in so-called “zero-field” nuclear magnetic resonance (ZFNMR) are nuclear quadrupole resonance (NQR) and antiferromagnetic NMR (AFNMR). This thesis provides a detailed theoretical summary of these resonances. Three economically important copper minerals, covellite (CuS), bornite (Cu_5FeS_4) and digenite (Cu_9S_5), were analysed with these techniques. Covellite was the exemplar for an alternative magnetic resonance pulse sequence that provides significant signals at low peak power. The sequence used is a single long pulse applied in the regime of large inhomogeneous broadening and short transverse and longitudinal decay times. Extensive experiments and modelling of Bloch equations provided a detailed description of the pulse sequence performance. The work has a practical application in that large-scale magnetic resonance instrumentation has limited peak power required to satisfactorily excite large volumes encountered in industrial applications. Single long pulses are observed to produce on-resonance signals at much lower powers than that is required for typical hard pulse sequences, providing an efficient alternative. A low temperature search for bornite was conducted to determine whether it was amenable to room temperature zero-field magnetic resonance. A spectrum was observed at 10 K and had multiple overlapped and broadened resonances over a 12 MHz frequency range. An attempt to model the spectra using the density functional theory was performed. The spectrum is interpreted as a result of complex mineral structure potentially relating to superstructures and magnetic ordering. Experiments at higher temperatures were not successful leading to the conclusion that room temperature resonances are unlikely to be observable. Digenite was studied in order to develop capability of room temperature measurement of this important mineral. Measurements were performed at 260 K and 280 K with the successful observation of resonances. The transverse decay times in digenite are very short. The technique of T_2 enhancement was attempted in order to lengthen the transverse decay times and provide a subsequent improvement of signal magnitude. The enhancement is produced by the application of a relatively weak static magnetic field to a pure quadrupole system. This improvement was observed in digenite however, room temperature observations were not possible due to the vary rapid reduction in decay time observed near room temperature. The experimental results in this thesis were obtained using an optimised home-built laboratory magnetic resonance spectrometer. Specific modifications for each of the experiments included probe construction for cryogenic applications, spectrometer electronics to allow detection over a very wide frequency band and inclusion of a weak static electromagnetic field with auxiliary coils. Overall, this work produced results that increased the knowledge, capability, and practical implementation for detection of resonances in the targeted copper minerals. Translation of the studied techniques into large scale analysers would enhance bulk mineral characterisation in mining and industrial applications

Solid-State NMR Spectroscopy of Unreceptive Nuclei in Inorganic and Organic Systems

Nuclei are termed unreceptive if they are not amenable to solid-state NMR (SSNMR) experimentation at standard magnetic field strengths due to (i) low natural abundances or dilution; (ii) low gyromagnetic ratios; (iii) inefficient longitudinal relaxation; (iv) large quadrupole moments; or (v) combinations of these factors. This thesis focuses on applying a variety of SSNMR methods to study unreceptive nuclei in a variety of systems. The first portion of this thesis focuses upon ultra-wideline (UW) SSNMR of three main group nuclei: 209Bi, 137Ba and 115In. 209Bi and 137Ba SSNMR were applied to a series of systems with important structural motifs, while 115In SSNMR was applied to systems with In in the +1 oxidation state. Extremely broad SSNMR spectra were acquired at field strengths of 9.4 and 21.1 T. In all cases, the electric field gradient (EFG) and the chemical shift (CS) tensor parameters obtained from these spectra are used to probe the metal coordination environments. These data are complemented by first principles calculations of the NMR tensors using molecular orbital (MO) and plane wave density functional theory (DFT) methods. The second portion of this thesis examines applications of SSNMR of unreceptive nuclei to some practical problems. First, 109Ag and 15N SSNMR experiments were performed to study silver supramolecular frameworks, and structural changes which occur upon their reactions with primary amines. 1H-109Ag cross polarization/magic-angle spinning (CP/MAS) NMR spectra were used to differentiate Ag sites, and 1H-15N CP/MAS NMR spectra provided measurements of 1J(109Ag, 15N) coupling constants, which are used to probe bonding Ag-N bonding. First principles calculations of silver and nitrogen CS tensors and 1J(109Ag, 15N) constants aided in formulating the structural models for the new materials. Second, 35Cl SSNMR spectra, single-crystal and powder X-ray diffraction data, and ab initio calculations were utilized to study HCl pharmaceuticals and some of their polymorphs. The sensitivity of the 35Cl EFG tensor parameters to subtle changes in the chlorine environments is reflected in the powder patterns, which can be used for structural interpretation, identifying and distinguishing polymorphs, and rapidly providing a spectral fingerprint of each pure pharmaceutical and its polymorphs

Natural Abundance 14N and 15N Solid-State NMR Studies of Organic, Biochemical, and Pharmaceutical Systems

Nitrogen is an important element in all areas of chemistry, biology, and materials science. However, it is challenging to probe directly with solid-state nuclear magnetic resonance (SSNMR). The commonly studied isotope, 15N (nuclear spin, I, = ½), has a low natural abundance (0.36%); therefore, 15N NMR experiments often require isotopically enriched samples. 14N (I = 1) has a high natural abundance (99.64%) but is a quadrupolar nucleus. 14N SSNMR spectra are generally very broad due to the moderate quadrupole moment of 14N. Ultra-wideline (UW) 14N SSNMR spectra have very low signal-to-noise, which complicates their acquisition; however, they are useful probes of molecular-level structure and dynamics, and do not require isotopic enrichment for their acquisition

NMR Crystallography of Disordered Cocrystals

Crystallographic disorder is common in the solid state but it is rarely investigated explicitly despite having a fundamental impact on the solid-state structure of a material. In this work, nuclear magnetic resonance (NMR) crystallography methods are utilised to achieve a detailed understanding of the structure and dynamics of solid organic systems containing disorder. Several new cocrystal systems are studied, each containing a topical drug molecule (caffeine, naproxen or furosemide) and each serving to demonstrate how NMR crystallography can be applied to a variety of structural questions. Hydrogen bonding motifs are identified using single crystal X-ray diffraction experiments, where possible, and are subsequently verified by solid-state NMR. Alternative hydrogen bonding models are ruled out by comparison of experimental solid-state NMR data with density functional theory calculated shieldings, and proton transfer can be investigated by monitoring the energy of the system with respect to proton position. In a particularly challenging case, 2D solid-state NMR experiments go some way to identify the hydrogen bonds in a system that cannot be crystallised. Dynamic disorder of fragments and solvent molecules are characterised by variable temperature solid-state NMR, including analysis of relaxation times to establish energy barriers and rates of motion. A mechanism of motion is also proposed for dynamic acetone molecules in a new cocrystal solvate, which is supported by good agreement between experimental and simulated 2H static NMR line shapes. Finally, the current limit of NMR crystallography is identified with respect to the reproducibility of calculated NMR parameters following geometry optimisation. It is shown that the geometry optimisation protocol does not affect standard NMR crystallography investigations pertaining to atom assignment, but it is significant for cases where very subtle structural features are probed, such as NMR linewidths. Overall, NMR crystallography investigations allow a deeper understanding of solid materials to be achieved than would be possible with any single technique and this work highlights the applicability of such methods to complex materials containing disorder

41st Rocky Mountain Conference on Analytical Chemistry

Final program, abstracts, and information about the 41st annual meeting of the Rocky Mountain Conference on Analytical Chemistry, co-sponsored by the Colorado Section of the American Chemical Society and the Rocky Mountain Section of the Society for Applied Spectroscopy. Held in Denver, Colorado, August 1-5, 1999

Intramolecular Hydrogen Bonding 2021

This book describes the results of both theoretical and experimental research on many topical issues in intramolecular hydrogen bonding. Its great advantage is that the presented research results have been obtained using many different techniques. Therefore, it is an excellent review of these methods, while showing their applicability to the current scientific issues regarding intramolecular hydrogen bonds. The experimental techniques used include X-ray diffraction, infrared and Raman spectroscopy (IR), nuclear magnetic resonance spectroscopy (NMR), nuclear quadrupole resonance spectroscopy (NQR), incoherent inelastic neutron scattering (IINS), and differential scanning calorimetry (DSC). The solvatochromic and luminescent studies are also described. On the other hand, theoretical research is based on ab initio calculations and the Car–Parrinello Molecular Dynamics (CPMD). In the latter case, a description of nuclear quantum effects (NQE) is also possible. This book also demonstrates the use of theoretical methods such as Quantum Theory of Atoms in Molecules (QTAIM), Interacting Quantum Atoms (IQA), Natural Bond Orbital (NBO), Non-Covalent Interactions (NCI) index, Molecular Tailoring Approach (MTA), and many others

59th Annual Rocky Mountain Conference on Magnetic Resonance

Final program, abstracts, and information about the 59th annual meeting of the Rocky Mountain Conference on Magnetic Resonance, co-endorsed by the Colorado Section of the American Chemical Society and the Society for Applied Spectroscopy. Held in Snowbird, Utah, July 22-27, 2018