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Applications of Fourier transform NMR spectroscopy : proton spin-lattice relaxation in organic molecules Preston, Caroline Margaret Callway

Abstract

This thesis describes a study of proton spin-lattice relaxation in organic molecules in solution, a study made possible largely by the development of Fourier transform n.m.r. (Spin-lattice relaxation, which concerns the transfer of energy between the spins and the molecular lattice, can occur by several different mechanisms, directly related to such lattice parameters as molecular geometry and motions.) A qualitative survey of some mono-saccharides and their derivatives revealed a stereospecific dependence of the proton spin-lattice relaxation times (T₁ values); further, the data strongly suggested that the dominant source of relaxation for the protons of an organic molecule in dilute degassed solution, in a magnetically inert, solvent, was the intramolecular dipole-dipole mechanism. The stereospecific differentials, which had clear diagnostic potential, extended to di- and polysaccharides, and an alkaloid, vindolene, but not to several high-molecular-weight polysaccharides. Study of two structurally-related groups of molecules all six-membered rings with known chair conformations in solution, showed that the proton relaxation could be analyzed as a sum of independent intramolecular dipole-dipole interactions between pairs of protons; moreover, these pairwise interactions were directly related to ratios of interproton distances. This model was further probed by two methods, selective deuteration and tailored excitation, and the results obtained by tailored excitation served to strengthen the assumption that the dominant source of relaxation was the intramolecular dipole-dipole mechanism. Other observations included the effect of changing temperature, concentration, and solvent, the extent of non-exponentiality in the decay curves, and the much more strongly non-exponential decay of some methyl protons, possibly due to cross-correlations. Finally, our new insight into proton relaxation patterns, combined with Fourier transform technology provided some practical methods for improving the high-resolution n.m.r. of large organic molecules, including manipulation of the naturally-occurring relaxation times by the addition of paramagnetic gadolinium (III) ions. This thesis also describes the empirical methods adopted, in the absence of a comprehensive theory, for determination of T₁ values in coupled, multi-spin systems, and finally, the preparation of some selectively deuterated sugar derivatives.

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