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Two labdane diterpenoids 8-hydroxymarrubiin and leonitin were isolated from Leonotis dysophylla (Benth.) and Leonotis leonitis respectively. Spectral studies of 8-hydroxymarrubiin, C₂₀H₂₈ O₅ʻ showed the presence of a β -substituted furan, a Ϫ-lactone, three tertiary methyl groups and tertiary hydroxyl group (s). The NMR spectrum of 8-hydroxymarrubiin and marrubiin C₂₀H₂₈O₄ʻ were almost identical with the exception of the C₁₇- methyl group which appeared as a singlet in 8-hydroxymarrubiin and as a doublet in marrubiin. The extra oxygen atom was therefore assumed to be present as a hydroxyl group substituted in the C₈₋ position. This was further confirmed by the formation of an epoxide and a Ϫό-dilactone. Leonitin, C₂₀H₂₈O₇ʻ was shown by spectral and chemical evidence to be a diterpenoid dilactone possessing an ester function and an ether linkage. Comparison of the NMR spectra of compound X and leonitin suggested that the acetoxy function occurs in the C₂₀- position. This was further supported by the formation of a 'Ϫό -dilactone. The absence of a β -furan moiety was apparent from chemical and spectral evidence, A structure for leonitin is proposed and aspects of its stereochemistry discussed.

The phase behaviour of methane at temperatures below 20°K is still not understood. There is recent evidence for a second phase transition at 8°K, in addition to the well known transition at 20.4°K. This second transition requires a long time to reach equilibrium. In our experiment, the nuclear magnetic resonance signal of the methane protons was monitored for a period of many hours at 1.2°K, in order to; determine whether a slow phase change occurred in the approach to equilibrium. This change would manifest itself in the width of the resonance line, which is sensitive to the local environment of the proton, and thus able to provide information about the crystal structure. No definite time variation of line width was observed. The line width increased by 15% from 4.2°K to 1.2°K. The second moment of the line also rose sharply as the temperature increased, indicating an increasing contribution to the spin-spin interaction from intra-molecular broadening. An approximate model yielded a correlation time for the intra-molecular term of the order of a microsecond. Significant differences between different methane samples indicate that sample preparation is important, and that the above results were for samples in some sort of metastable phase. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Selective excitation pulsed NMRON, CW-NMRON and Thermal NMR methods have been used to study the low temperature ⁵⁴Mn nuclear spin-lattice relaxation mechanisms in magnetic insulators. The selective single and double quantum excitation sequences have been used for the first time in NMRON to obtain single and double quantum rotation patterns, Free Induction Decays, Hahn spin echoes and pulsed T₁ measurements. Two insulating magnets have been studied; MnCl₂.4H₂O and Mn(COOCH₃ )₂ .4H₂O. In the ⁵⁴Mn-MnCl₂ .4H₂O system the temperature dependence of the ⁵⁴Mn spin-lattice relaxation time at zero field was measured between 35 mK and 90 mK and it was found that the dominant relaxation process between 65 mK and 90 mK is an electronic magnon Raman process and below 65 mK a direct relaxation process dominates. Single and double quantum Free Induction Decays and Hahn spin echoes have been used to determine the magnitude and nature of the spin-spin relaxation mechanism for ⁵⁴Mn oriented in MnCl₂.4H₂O at zero applied field. NMRON was observed for the first time in the paramagnetic phase of MnCl₂.4H₂O. The resonance lines are inhomogeneously broadened and 300 kHz wide. A value of <⁵⁴AS>/h=-513.6(3) MHz has been determined for the paramagnetic phase hyperfine coupling constant, and this value has been used to determine the zero point spin deviation of the antiferromagnetic phase. The field and temperature dependence of the ⁵⁴Mn T₁ was measured for values of field above the spin flop paramagnetic phase transition and a field dependent T₁ minimum was discovered at Ba=2.64 T. For the ⁵⁴Mn-Mn(COOCH₃) .4H₂O system two ⁵⁴Mn resonances have been observed and the value of the hyper fine coupling constants for the two sites were found to be <⁵⁴AS>/h=-435 (1) MHz for the ⁵⁴Mn1 site and <⁵⁴AS>/h=-478(1) MHz for the ⁵⁴Mn2 site. The high field spin-lattice relaxation behavior has also been investigated and a T₁ minimum at Ba =2.74 T analogous to that observed in MnCl₂ .4H₂O was discovered. A Hahn echo study of the low field single quantum spin-spin relaxation processes has been performed and anomalous behavior of the spin echo amplitude revealed. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

A number of solid substances ware examined by nuclear magnetic resonance methods with a view to investigating possible molecular motion. The possibility of using the adiabatic rapid passage technique as a method for investigating the molecular motion in the solid state was studied. Two systems, namely benzene and furan were studied. It was found that spin-lattice relaxation times and in some cases, second moments could be obtained using the adiabatic rapid passage technique; the results obtained were in good agreement with values obtained using standard techniques. Also it was found that the presence of a certain type of molecular motion severely affects the shape and amplitude of the adiabatic rapid passage signal. A number of charge-transfer complexes were investigated using standard broadline (1)H and (19)F nuclear magnetic resonance techniques. Some strong charge-transfer complexes studied in this manner include: a number of amine complexes of BF(3) and some halogen complexes of trimethylamine. Second moment and linewidth changes with temperature were interpreted in terms of reorientations of molecular groups within the complexes. Also studied were a number of weak complexes of benzene. Linewidth, second moment, and also spin-lattice relaxation time measurements showed that the benzene rings were reorientating about their hexad axes at frequencies greater than about 10(5) sec.(-1) at temperatures above 120°K. The activation energies for this motion depended strongly on the environment of the benzene ring in the complex. A study of some arene-chromium-tricarbonyl compounds indicated that the motional properties of the arene rings in the complexes resembled very much the motional properties of the free ring compounds; this suggests that specific bonding effects are relatively inimportant. Finally, linewidth, second moment and spin-lattice relaxation time measurements are reported for some soap systems, the alkali metal stearates and oleates. Transition temperatures observed could in some cases be correlated with values obtained by different methods. Some motional models were suggested in order to explain decreases in linewidths and second moments with increasing temperature. It was shown that methyl group rotation provides a relaxation mechanism in the low temperature phase for all the soaps studied. Activation energies found for this process ranged from 1.8-2.5 Kcal/mole. The effect of thermal history on phase transitions in the alkali metal stearates was also investigated. Thermograms were obtained using samples with different thermal histories using a differential scanning calorimeter. It was found that the thermal history of a sample may affect transition temperatures, and also the absence of presence of some transitions. / Science, Faculty of / Chemistry, Department of / Graduate

The spin-lattice relaxation time T₁ has been measured in the temperature range 1.2 to 55°K at 28.5 mcs. for the proton resonance, and at 4.4 mcs for the deuteron resonance using N.M.R. pulse techniques. The proton T₁ has been measured for CH₄, CH₃D, CD₃H, 50%CH₄-50%Kr, 90%CH₄-10%Kr, 67%CD₄-33%CH₄, 10%CD₄-90%CH₄, and also for CH₄ at 4.4 mcs. The deuteron T₁ has been measured for CD₄, CD₃H, and 67%CD₄-33%CH₄. It is found that a drastic change in the temperature dependence of T₁ occurs in the temperature region below the phase transitions and that at most of the phase transition temperatures there is either a discontinuous change in T₁ or a change in the slope of T₁ versus T. A minimum in T₁ is found at low temperatures for all the systems studied. An analysis of the data based on conventional N.M.R. theory shows in most cases that the correlation time Ƭc α T¯⁷ in the neighbourhood of 20°K, and that Ƭc is almost independent of temperature near 1.2°K. It is postulated that phonon-molecular interactions, involving direct and Raman processes, can account for the temperature dependence of Ƭc. The values of T₁ at the minimum are completely determined by conventional theory. In most cases, however, the predicted values are of the order of 20 times too short. An unexplained minimum in T₁ was observed in CH₄, CH₄-CD₄, and CH₄-Kr mixtures above the upper phase transitions. To investigate the origin of some of the inadequacies of the conventional theory, the two energy level scheme proposed by Colwell, Gill, and Morrison (1965) is used, where each of the two levels may be degenerate. Simple rate equations are used to calculate the conditional probabilities and the correlation functions for the two level model. It is found that the effective interaction strength is temperature dependent, that the correlation function can be described by a simple exponential under certain conditions, and that the interaction strength has no simple relationship with the classical value. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The ¹H and ¹⁹F nuclear magnetic resonance (n.m.r.) spectra of a series of hexopyranosyl and pentopyranosyl fluorides were examined in some detail. The ¹H n.m.r. analyses (first order) enabled the gross molecular conformations to be determined and at the same time provided an opportunity to check the anomeric assignments previously made through application of other techniques. The¹⁹F n.m.r. parameters were then considered and several steric dependencies were observed. The ¹⁹F chemical shifts are controlled, in part, by the spatial orientation of the fluorine, but also, by the orientation of remote acetate groups or other substituents. Rules were subsequently developed for predicting the ¹⁹F chemical shifts for axial fluorines attached to C₁ of a pyranose carbohydrate. These rules were used to predict the ¹⁹F chemical shifts of the remaining "rare" giycopyranosyl fluorides. It was also noted that the ¹⁹F-¹H couplings resemble the ¹H-¹H couplings in that J(trans) > J(gauche). Moreover these ¹⁹F-¹H couplings are several times larger than the corresponding ¹H-¹H couplings and are also more sensitive to their precise steric environment. This study led to the observation that the β-D-anomers of the pentopyranosyl fluorides exist in the alternate chair form (¹C₄), which allows the fluorine atom to adopt an axial orientation. Subsequent to these initial n.m.r. investigations, a new reaction sequence was developed for the addition of "XF" to alkenes, Whereas this reaction sequence (AgF and X₂) results only in trans addition to cyclohexene (courtesy of D.L. Jones); it results in both cis and trans additions to cyclic vinyl ethers, although the trans adduct(s) always predominate: a possible reaction mechanism is discussed. This reaction sequence (X = I, Br,CI) was first applied successfully to D-glucal triacetate and then extended to other α,β-unsaturated cyclic ethers. The reaction products to D-glucal triacetate were for the most part crystalline. These 2-deoxy-2-halo-D-hexo-pyranosyl fluorides proved to be excellent models for extending previously observed electronegative dependencies. Moreover, when related to the 2-deoxy-D-glycopyranosyl fluorides, these compounds revealed that the orientation of remote substituents must play an important role in - determining ¹⁹F chemical shifts. Finally, a brief introduction is given to the utilization of both the "selective irradiation" and the "spin-tickling" methods for determining the relative signs of proton-proton coupling constants of carbohydrate derivatives. For most saturated carbohydrates the vicinal ¹H-¹H couplings have been found to be positive and the geminal couplings are negative in sign. For unsaturated derivatives both vicinal and geminal couplings are positive. The signs of the couplings of an episulphide derivative are the same as those of the saturated carbohydrates, whereas, the signs for an epoxide resemble those of unsaturated carbohydrates. These findings arc in accord with previous experimental and theoretical results. It has been demonstrated that the signs of long-range ( ⁴J) couplings of saturated derivatives exhibit a regular stereospecificity: those between two equatorially oriented protons (⁴ J, ₑ,ₑ) being positive and those between an axial and an equatorial proton (⁴Jₑ,ₐ ) being negative in sign. These findings are contrasted with the previous theoretical predictions and their potential as an adjunct to stereochemical studies is discussed. For unsaturated carbohydrate derivatives the long-range couplings are all negative in sign. The "selective irradiation" and "spin-tickling" homonuclear magnetic double resonance (n.m.d.r.) techniques have been used to determine the relative signs of the vicinal and geminal ¹⁹F- ¹H couplings of a series of glycopyranosyl fluorides. In all cases these two couplings have the same relative signs. Heteronuclear decoupling experiments on 2-bromo-2-deoxy-α-D-mannopyranosyl fluoride triacetate showed that for this derivative the vicinal coupling between H₁ and H₂ and the geminal coupling between H₁ and F₁ have the same relative signs. On this basis an absolute positive sign was assigned to both vicinal and geminal ¹⁹F- ¹H couplings. / Science, Faculty of / Chemistry, Department of / Graduate

The proton magnetic resonance absorption of the C-form of stearic acid has been studied, over the temperature range 77°K to 3^2.5°K. The broadline spectra are decomposed into a wide and narrow component for solid and liquid regions in the lattice respectively. The appearance of the narrow component on the usual broadline spectrum at temperatures greater than room temperature is interpreted to be caused, by large liquid regions in the lattice. The number and size of these defects increase with increase in temperature and. with the incorporation of various types of defects in the solid lattice The spin-lattice relaxation times, T₁, were measured by adiabatic rapid, passage. The results indicate that the principal zeeman energy-thermal bath coupling occurs through methyl group reorientations. An activation energy of 2.2 kcalories/mole for methyl group rotation about the C₃ axis in solid stearic acid and a rotation frequency of 4 x 10⁵ sec⁻¹ at 77°K were determined from the T₁ data. / Science, Faculty of / Chemistry, Department of / Graduate