A gas of atomic hydrogen confined at and below 1K in a sealed cell has been studied by magnetic resonance at the zero-field hyperfine transition frequency of 1420 MHz. A review is presented of magnetic resonance theory for a two level system, with emphasis on determination of the absolute magnetization by two methods: calibration of the spectrometer sensitivity and by use of the radiation damping time constant. Measurements at 1K on a gas at low density, 10¹¹<n[sub=H]<5x10¹² cm⁻³, in the saturated ⁴He vapour density have yielded the rate for the reaction H+H+He￫H₂+He, the diffusion constant and pressure shift of the hyperfine transition for H interacting with the He gas, and the cross-section for spin exchange relaxation. At temperatures below 1K, measurements of the frequency shift and effective recombination rate for H adsorbed on the He film have yielded values of the binding energies for H on ³He and for H on ³He as well as the hyperfine transition frequency shift and surface recombination rate for H adsorbed on each of these surfaces. The binding energies are found to be 1.15(5) K for H on ³He and 0.42(5) K for H on ³He. Measurements have been carried out at temperatures as low as 162 mK for H on ³He and 65 mK for H on ³He. lineshape for H atoms undergoing occasional sticking events on the helium surface. This model has been applied to frequency shift and transverse relaxation data at low temperatures to yield sticking probabilities of 0.046(5) for H on ³He and 0.016(5) for H on ³He. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The work described herein falls into three major categories: synthesis of model glycolipids, NMR studies on'model glycolipid'micelles, and application of 2D-NMR spectroscopy in spectral assignment. Three synthetic routes, namely the glycosidation reaction, reductive amination reaction, and amide bond formation have been investigated in relation to their efficiency and convenience in coupling carbohydrates with aliphatic chains. The reaction of amide bond formation was found to be a superior method over the others for the preparation of long alkyl chain derivatives. ¹H-NMR spectroscopy has been utilized to study and detect the micelle formation by the model glycolipids. The studies described illustrate that the ¹H spin-lattice relaxation rate (R-₁) is well suited for the determination of critical micelle concentration providing it is sufficiently high. The contrasting behaviour of R-₁ of the anomeric proton (H-1) of n-octyl β-D-glucoside in relation to that of H-2 and w-CH₃ upon micellization, has been tentatively attributed to the conformational changes accompanying micelle formation. The observed upfield shift of the ¹³C resonances of the alkyl chain has been explained as being due to the increased proportion of trans conformers in the micellar state. The question of the downfield ¹³C shift observed for the sugar resonances has been discussed. Study of N-alkyllactobionamides with ¹H-NMR proved to be difficult due to their extremely low critical micelle concentrations. Application of 2D J-resolved spectroscopy and spin-echo correlated spectroscopy (SECSY) in spectral assignment of unprotected sugar derivatives has been demonstrated. Using above techniques, complete assignment of the sugar region of n-octyl β-D-glucoside and N-hexyllactobionamide has been achieved. / Science, Faculty of / Chemistry, Department of / Graduate

Nuclear magnetic resonance absorption and spin-lattice relaxation time measurements have been carried out on the tropolone salt of t-butylamine, (CH₃)₃CN⁺H₃Tr⁻ (Tr ⁻= tropolonate ion, C₇H₅0₂⁻), the choline salts, (CH₃)₃NCH₂CH₂OH. X⁻ (x⁻ = Cl⁻, Br , I , ClO₄ ⁻) and the trimethylamine-phosphorous penta-fluoride adduct, (CH₃)₃NPF₅, in order to study molecular motion and phase transitions in these systems in the solid state. Activation energies and rate parameters associated with the motional processes are reported. Proton magnetic resonance (pmr) absorption second moments and proton spin-lattice relaxation times in the Zeeman frame (T₁) in the temperature range 66K - 425K for the solid (CH₃)₃CNH₃Tr⁻ show that the molecule is rigid on the nmr timescale at the lowest temperature studied, while at higher temperatures rotation of methyls about their C₃ symmetry axes is found to set in first, followed by an additional composite motion involving reorientation of both the t-butyl group and the NH₃ group about the C-N bond. A proton study in the partially deuterated (-ND₃) analogue has enabled the relaxation effects of the latter two motions to be separated, and, by fitting the T₁, data for the two compounds to appropriate relaxation rate expressions, activational energy barriers for the abovementioned motional processes have been determined. It has also been suggested that the t-butyl group and the NH₃ group rotate independently about the C-N bond rather than as one unit. Proton spin-lattice relaxation time measurements in both the Zeeman and rotating frames of reference (T₁ and T₁[sub p]) for the four choline salts and second moments of the pmr absorption for the perchlorate have enabled the following motional processes to be identified: (i) rotation of the methyl groups at low temperatures followed successively by, (ii) the onset of motion of the NMe₃ moiety about the long chain C-N axis (denoted C₃), (iii) general reorientation of the whole choline cation, (iv) additional slow motion of the long chain (CH₂CH₂OH in the case of the chloride and bromide, and (v) diffusion of the choline ion in the case of the iodide and perchlorate. From a quantitative analysis of the and data, activation energies for the above types of motion are determined. A crystal-crystal phase transition known to occur at 353, 364 and 362K in the chloride, bromide and iodide, respectively, has been confirmed. A similar transition has been discovered in the perchlorate, and is found to occur at a much lower temperature (272K). Evidence has also been presented for a further crystal-crystal phase transition in choline iodide at 430K, at which point a "quenching" of the diffusional process is found in this structure. In the adduct (CH₃)₃NPF₅, studies of proton and fluorine nmr absorption spectra and measurements have shown that (i) at 4.2K the molecule is 'rigid1, (ii) C₃ reorientation of one of the methyls and reorientation of the PF₅ group about the P-N bond cause a ¹H and ¹⁹F nmr line narrowing, (iii) this is followed by the C₃ rotation of the other two methyl groups together with the rotation of the (CH₃)₃N group about the P-N bond. These are confirmed by a successful simulation of the observed pmr lineshapes at 4.2K and at 77K. The proton and fluorine T₁ data show the ¹H and ¹⁹F spins to be strongly coupled. A study of fluorine T₁ in the fully deuterated compound, (CD₃)₃NPF₅ has enabled the analysis of the overall T₁ data to be simplified. The observed trends in the T₁ data are seen to be well explained by the theory for a coupled spin system of two unlike spins. / Science, Faculty of / Chemistry, Department of / Graduate

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