No description available.

1- The synthesis of unusually substituted halo-fluoroheterocycles has been achieved. 2,4,6-Tribromo-3,5-difluoropyridine and 4-bromo-2,3,5,6-tetrafluoropyridine were prepared from pentafluoropyridine, aluminium bromide and hydrogen bromide Reactions with lithium halides allowed the preparation of 4-iodo-2,3,5,6- tetrafluoropyridine, 4-iodo-2,6-dibromo-3,5-difluoropyridine, 4-iodo-2,3,5,6- tetrafluoropyridine and 4-chloro-2,6-dibromo-3,5-difluoropyridine.2- Reactions of 2,4,6-tribromo-3, 5-difluoropyridine with nucleophiles showed that selective substitution at the C-F centre can be achieved using hard (sodium ethoxide, phenoxide) nucleophiles.3- Lithium mediated reactions of 2,4,6-tribromo-3,5-difluoropyridine allowed selective functionalisation of the 4-position; lithium-halogen exchange occurred exclusively at this position giving a stable lithium derivative, which was successfully trapped with a range of electrophiles (trimethylsilyl chloride, acid chlorides).4- Palladium mediated couplings of 2,4,6-tribromo-3,5-difluoropyridine with a range of substituted phenylacetylenes was successful and the 2- and 6-positions were the most activated sites. Coupling with boronic acids gave the disubstituted or trisubstituted products; in the case of trisubstitution, all three positions (2-,4- and 6-) were activated towards coupling. 2,4,6-Tribromo-3,5-difluoropyridine formed a stable zinc derivative, which was coupled with iodoaromatics (iodobenzene, diiodobenzene).

This Thesis describes the development of a broadband pulsed NMR spectrometer, based on a sensitive DC SQUID amplifier with wideband electronics, to observe directly the free precession of nuclear spins in bulk metallic samples (with broad NMR linewidths) at Larmor frequencies cß/2 ,r below 1 MHz. The sample is located inside a pickup coil, which forms a superconducting flux transformer with the input coil of the SQUID. The SQUID amplifier operates in a flux-locked-loop (FLL), hence it is sensitive to signals from DC up to the bandwidth of the FLL electronics. A modified commercial DC SQUID amplifier, with modulated feedback electronics, observed NMR signals from bulk platinum samples (T2 - 1.1 ms), at 1.5 K. The SQUID amplifier had a 50 kHz bandwidth, a dead-time - 50 μs, and a coupled energy sensitivity cc - 500h. The measurements showed that it is important to minimise the time-constant of eddy current decay in the sample, which scales with r2, as expected, where r is the sample dimension. A DC SQUID amplifier with additional positive feedback and wideband electronics configured using the direct offset integration technique, observed NMR signals from a bulk aluminium sample (T2 - 30 μs) at 20 mK. This SQUID amplifier had a 7.5 MHz bandwidth, the dead-time was 55 μs for small transmitter pulses and e,; - 600h. The use of a strongly coupled input coil with the SQUID necessitated damping across the coil to smooth out the SQUID flux-voltage characteristicThe NMR measurements showed that eddy current decay is less important if the NMR signal size is enhanced by cooling the sample. Measurements also confirmed that the NMR signal from bulk metal is proportional to 4c0, and that a reasonable estimate of the signal size is made by assuming the signal is due to spins within half the skin-depth of the surface

No description available.

A model for studying magnetisation during FC-PEDRI experiments has been developed and verified experimentally. The optimum EPR irradiation frequency, which maximises the Overhauser enhancement while minimising the RF power deposition in sample, was investigated using the FC-DNP technique. Experiments to verify the model were carried out in the EPR frequency range between 20 and 180 MHz and with different applied ERP RF power for free radicals with different EPR linewidths. The optimum EPR irradiation frequency was found to be dependent on the applied power level and on the EPR linewidths. Methods to obtain NMR images with T₁-weighted were also investigated. The most successful method was found to be a combination of Inversion Recovery and FC-PEDRI. This technique yields both high <I>T</I>₁ contrast between samples and good contrast between images with different <I>T</I>₁ values. The free radical distribution is recorded with the same SNR as those collected with the conventional FC-PEDRI techniques. Rapid imaging of FC-PEDRI has also been studied. The number of EPR irradiation periods can be optimised to give a good free radical distribution image while reducing the power deposition in the sample. With 4 EPR irradiation periods, the difference image displays the free radical distribution comparable with the conventional technique (64 EPR irradiation periods). The power deposition in the sample is reduced by the factor of 16 and acquisition time is reduced by a factor of 4. Finally, inversion recovery spin preparation has been added to the hybrid fast sample. It was found that an IR hybrid FISP FC-PEDRI pulse sequence could be used to generate <I>T</I>₁ contrast in images which show similar detail and intensity of the free radical distribution as those obtained using hybrid FISP-PEDRI.

No description available.

The research work reported in this thesis deals with the development of novel NMR experimental techniques for the spectroscopic discrimination of enantiomers dissolved in a chiral liquid crystalline medium. The information on the chemical shifts and coupling constants pertaining to each enantiomer has been derived on the investigated chiral molecules. The enantiomeric excess (ee), a parameter which is of profound importance in pharmaceutical industry and in asymmetric synthesis, has also been measured. A special attention is paid to the use of high sensitivity of H NMR for chiral discrimination. Typical analyses of H NMR spectra are severely hindered due to enormous spectral inhomogeneous broadening arising from too many unresolved transitions, in addition to superposition of spectra from both the enantiomers. Therefore, the major part of the work is focused on the design and application of pulse sequences to overcome many of these drawbacks. This helps to achieve very high resolution, discerning of overlapped transitions, identification of resonances pertaining to each enantiomer and simplification of the spectrum for easy extraction of spectral parameters, in addition to the accurate measurement of ee. Initially a brief discussion is provided on enantiomers, diastereomers, basic principles of NMR spectroscopy, the several interaction Hamiltonians responsible for yielding the NMR spectra, introduction to product and polarization operator formalisms that gives insight into the spin dynamics for designing appropriate two-dimensional (2D) NMR experiments. This sets the foundation to understand the complex multiplet structures of the diagonal peaks and cross peaks in the resulting 2D spectrum. Subsequently, a brief introduction is given for the available techniques for NMR spectroscopic discrimination of enantiomers in isotropic medium, where only chemical shifts are employed as a measurable parameter. The limitations of these techniques are circumvented by the introduction of other anisotropic NMR parameters, such as homo-and hetero-nuclear dipolar couplings, quadrupolar couplings and chemical shift anisotropies. To achieve this goal the enantiomers are dissolved in weakly aligning chiral liquid crystalline (CLC) medium. To understand this, a general introduction to liquid crystals and their utility as an alignment medium in NMR spectroscopy and the anisotropic interactions affecting the NMR spectrum has also been provided. The preparation of the CLC phase of Poly-γ-Benzyl-L-Glutamate (PBLG) employed in the present study and its orientational behaviour has been discussed. The detection of NMR spectra of various nuclei and the interaction parameters utilized for chiral discrimination will be enumerated. A brief summary of the experiments employed for the spectral analyses of the enantiomers dissolved in PBLG will also be presented.

No description available.

No description available.

No description available.