Multi-Acquisition and Multi-Dimensional Earth's Field Nuclear Magnetic Resonance Spectroscopy

<p>In this thesis we investigate the ways in which the sensitivity, resolution and overall performance of an Earth's field NMR system can be improved without significantly compromising its simplicity, portability or affordability. We investigate the limits of the information obtainable using this device and present a range of methods for calculating and analyzing NMR spectroscopy experiments detected in the Earth's magnetic field. We demonstrate significant improvements in the performance of a commercial Earth's field NMR device, the Terranova-MRI, through several apparatus developments. First-order shimming is added to the system in order to counter any local inhomogeneity of the Earth's field. The spectral resolution of the instrument is further improved through the introduction of a field locking system to counter the natural temporal drift in the magnitude of the Earth's magnetic field. External noise interference is reduced through the use of Faraday screening, effectively increasing the signal-to-noise ratio (SNR) performance of the device. We explore three signal enhancement methodologies for optimizing the SNR performance of the system. Prepolarization, with an electromagnet as well as a permanent magnet array, is considered and compared to dynamic nuclear polarization (DNP) and hyperpolarization via optical pumping. We present a detailed theoretical discussion of DNP in low-fields and demonstrate the application of this technique for signal enhancement in EFNMR. An apparatus for performing DNP in the Earth's field is presented and optimized. A density matrix approach to simulating one- and two-dimensional Earth's field NMR experiments is presented. These numerical simulations, along with a perturbation theory approach to calculating one-dimensional EFNMR spectra of tightly coupled heteronuclear systems, are explored and compared to experimental spectra of the tetrahydroborate and ammonium ions. These systems are of particular interest for NMR detected in the Earth's field because they contain strongly coupled nuclei of differing spin, a situation previously unexplored in the literature. Multi-dimensional Earth's field NMR spectroscopy methods, in particular the correlation spectroscopy (COSY) experiment, are implemented and optimized through the use of shimming, field stabilization and noise screening. The 2D COSY spectrum of monofluorobenzene is analyzed and compared to calculated spectra in order to determine the indirect spin-spin coupling constants of this molecule in the Earth's magnetic field. A 2D COSY spectrum of 1,4-difluorobenzene is also presented and compared to simulation. The SNR performance of COSY in the Earth's field is greatly improved through the use of DNP for signal enhancement. A high-quality, 2D COSY EFNMR spectrum with DNP acquired from 2,2,2- trifluoroethanol is presented and compared to simulation. The particular features of this spectrum, which result from the use of DNP for signal enhancement, are discussed with reference to a density matrix simulation and to a one-dimensional spectrum calculated using perturbation theory. The strong indirect spin-spin coupling regime in fields weaker than the Earth's magnetic field is explored through exact calculations and density matrix simulations of a 13C-enriched methyl group. A novel multi-dimensional EFNMR method for observing such spectra is discussed. This experiment allows for the resolution of strongly coupled NMR spectra both in the Earth's magnetic field, in the directly detected domain, and in weaker fields, in the indirectly detected domain. In the final section of this thesis, residual dipolar coupling is observed by EFNMR for the first time in a system of poly-[gamma]-benzyl-L-glutamate (PBLG) in dichloromethane. The form of the EFNMR spectrum of this liquid crystalline system is discussed and compared to equivalent high-field (9.4T) spectra.</p>

Multi-Acquisition and Multi-Dimensional Earth's Field Nuclear Magnetic Resonance Spectroscopy

<p>In this thesis we investigate the ways in which the sensitivity, resolution and overall performance of an Earth's field NMR system can be improved without significantly compromising its simplicity, portability or affordability. We investigate the limits of the information obtainable using this device and present a range of methods for calculating and analyzing NMR spectroscopy experiments detected in the Earth's magnetic field. We demonstrate significant improvements in the performance of a commercial Earth's field NMR device, the Terranova-MRI, through several apparatus developments. First-order shimming is added to the system in order to counter any local inhomogeneity of the Earth's field. The spectral resolution of the instrument is further improved through the introduction of a field locking system to counter the natural temporal drift in the magnitude of the Earth's magnetic field. External noise interference is reduced through the use of Faraday screening, effectively increasing the signal-to-noise ratio (SNR) performance of the device. We explore three signal enhancement methodologies for optimizing the SNR performance of the system. Prepolarization, with an electromagnet as well as a permanent magnet array, is considered and compared to dynamic nuclear polarization (DNP) and hyperpolarization via optical pumping. We present a detailed theoretical discussion of DNP in low-fields and demonstrate the application of this technique for signal enhancement in EFNMR. An apparatus for performing DNP in the Earth's field is presented and optimized. A density matrix approach to simulating one- and two-dimensional Earth's field NMR experiments is presented. These numerical simulations, along with a perturbation theory approach to calculating one-dimensional EFNMR spectra of tightly coupled heteronuclear systems, are explored and compared to experimental spectra of the tetrahydroborate and ammonium ions. These systems are of particular interest for NMR detected in the Earth's field because they contain strongly coupled nuclei of differing spin, a situation previously unexplored in the literature. Multi-dimensional Earth's field NMR spectroscopy methods, in particular the correlation spectroscopy (COSY) experiment, are implemented and optimized through the use of shimming, field stabilization and noise screening. The 2D COSY spectrum of monofluorobenzene is analyzed and compared to calculated spectra in order to determine the indirect spin-spin coupling constants of this molecule in the Earth's magnetic field. A 2D COSY spectrum of 1,4-difluorobenzene is also presented and compared to simulation. The SNR performance of COSY in the Earth's field is greatly improved through the use of DNP for signal enhancement. A high-quality, 2D COSY EFNMR spectrum with DNP acquired from 2,2,2- trifluoroethanol is presented and compared to simulation. The particular features of this spectrum, which result from the use of DNP for signal enhancement, are discussed with reference to a density matrix simulation and to a one-dimensional spectrum calculated using perturbation theory. The strong indirect spin-spin coupling regime in fields weaker than the Earth's magnetic field is explored through exact calculations and density matrix simulations of a 13C-enriched methyl group. A novel multi-dimensional EFNMR method for observing such spectra is discussed. This experiment allows for the resolution of strongly coupled NMR spectra both in the Earth's magnetic field, in the directly detected domain, and in weaker fields, in the indirectly detected domain. In the final section of this thesis, residual dipolar coupling is observed by EFNMR for the first time in a system of poly-[gamma]-benzyl-L-glutamate (PBLG) in dichloromethane. The form of the EFNMR spectrum of this liquid crystalline system is discussed and compared to equivalent high-field (9.4T) spectra.</p>

Theoretical Investigation of Glycine Micro-Solvated. Energy and NMR Spin Spin Coupling Constants Calculations

Glycine in its neutral form can exist in the gas phase while its zwitterion form is more stable in water solution, but how many waters are actually necessary to stabilize the zwitterionic structure in the gas phase? Are the intramolecular isotropic spin spin coupling constants sensitive enough to accuse the change in the environment? or the conformer observed? These and related questions have been investigated by a computational study at the level of density functional theory employing the B3LYP functional and the 6-31++G**-J basis set. We found that at least two water molecules explicitly accounted for in the super-molecule structure are necessary to stabilize both conformers of glycine within a water polarizable continuum model. At least half of the SSCCs of both conformers are very stable to changes in the environment and at least four of them differ significantly between Neutral and Zwitterion conformation.

Theoretical Investigation of Glycine Micro-solvated. Energy and NMR Spin Spin Coupling Constants Calculations

Glycine in its neutral form can exist in the gas phase while its zwitterion form is more stable in water solution.But how many waters are actually necessary to stabilize the zwitterionic structure in the gas phase? Are the intramolecular isotropic spin spin coupling constants sensitive enough to accuse the change in the environment? or the conformer observed? These and related questions have been investigated by a computational study at the level of density functional theory employing the B3LYP functional and the 6-31++G**-J basis set. We found that at least two water molecules explicitly accounted in the super-molecule structure are necessary to stabilize both conformers of glycine within a water polarizable continuum model. At least half of the SSCC&rsquo;s of both conformers are very stable to changes in the environment and at least four of them differ significantly between Neutral and Zwitterion conformation.

1,2-Dihydro-1,3,2-diazaborinine tautomer as an electron-pair donor in hydrogen-bonded complexes

Ab initio MP2/aug’-cc-pVTZ calculations have been carried out to investigate 1,2-dihydro-1,3,2-diazaborinine:HX complexes for HX = H+, HF, HCl, H2O, HCN, NH3, HCP, and HCCH. Most complexes are stabilized by linear, traditional hydrogen bonds except for those with H2O and NH3 which have bridging structures and nonlinear hydrogen bonds. H-atom transfer from N to B can occur in complexes with HF and HCl, with formation of a traditional F-H…N and a proton-shared Cl…H…N bond. The binding energies of the uncharged complexes range from 25 to 88 kJ.mol–1. Spin-spin coupling constants have been used to characterize these hydrogen-bonded complexes. Des calculs ab initio MP2/aug'-cc-pVTZ ont été effectués pour étudier les complexes 1,2-dihydro-1,3,2-diazaborinine:HX pour HX = H+, HF, HCl, H2O, HCN, NH3, HCP et HCCH. La plupart des complexes sont stabilisés par des liaisons hydrogène traditionnelles, linéaires, à l'exception de celles avec H2O et NH3 qui ont des structures pont et des liaisons hydrogène non linéaires. Le transfert de l'atome d'hydrogène de N à B peut se produire dans des complexes avec HF et HCl, avec formation d'une liaison F-H···N traditionnelle et d'une liaison Cl···H···N avec un proton comparti. Les énergies de liaison des complexes non chargés vont de 25 à 88 kJ·mol–1. Des constantes de couplage spin-spin ont été utilisées pour caractériser ces complexes à liaison l'hydrogène.