Le travail de recherche présenté concerne l'étude théorique des molécules LuBr et LuI qui sont considérées comme des molécules d'intérêt astrophysique. Les calculs ont été effectués en utilisant les méthodes de chimie quantique usuelles pour ce type de molécules comme les méthodes Hartree-Fock Self-Consistent Field (SCF) , Complete Active Space Self-Consistent Field (CAS-SCF) et les méthodes d'interaction de configurations multi-référence (MRCI) avec la correction de Davidson. Pour cela, la chaîne de programmes MOLPRO a été utilisée. Les courbes d'énergie potentielle ont été déterminées pour un grand nombre d'états électroniques dans un premier temps sans considérer l'interaction spin-orbite puis en l'incluant. Les constantes spectroscopiques (position d'équilibre (Re), l'énergie d'excitation (Te) et la fréquence harmonique (we) ont été déduites et comparées aux rares résultats expérimentaux disponibles dans la littérature pour ces deux molécules. / This work concerns the theoretical study of the electronic structure of two molecules of astrophysical interest, LuBr and LuI. Ab-initio methods have been used as Self-Consistent Field (SCF) method, Complete Active Space Self-Consistent Field (CAS-SCF) method and MRCI calculations including Davidson correction. Calculations have been performed through MOLPRO. Potential energy curves have been determined for numerous electronic states first without the spin-orbit coupling and afterwards with this interaction. Spectroscopic constants as equilibrium position (Re), excitation energy (Te) and harmonic frequency (we) have been deduced and compared with available experimental data for these two molecules.

The design and synthesis of stable organic radicals with delocalized spin density distribution and low energy optical and redox processes is central to the development of magneto-conducting materials. Towards this end, a generalized synthetic methodology has been developed allowing for the synthesis of a series of annelated benzonitronyl nitroxide (BNN) radicals. The BNN radicals exhibited remarkably low reduction potentials (~0.0 V vs SCE) and a near-infrared absorption attributed to a HOMO–SOMO charge-transfer excitation. The annelated BNN radicals were found to be less stable than the closely related tetramethyl nitronyl nitroxide radicals, particularly in solution. A series of π-delocalized and heteroaromatic radicals were synthesized to determine if the instability was due to the delocalization of electron density onto the carbon skeleton or the low reduction potential. DFT calculations with the EPR-II basis gave rise to calculated electronic structures that were consistent with EPR spectroscopy and suggested changes in spin density distribution are occurring with perturbation of the annelated ring. Cyclic voltammetry revealed the heteroaromatic and π-delocalized radicals had reduction potentials lower than BNN, with some systems reducing at potentials of 0.2 V vs SCE, comparable to that of 7,7,8,8-tetracyanoquinodimethane. The distribution of spin throughout the molecular framework and the low reduction potential of the annelated nitronyl nitroxide radicals were both found to contribute to the lowered stability of the annelated nitronyl nitroxides relative to the far less redox active tetramethyl nitronyl nitroxides. The low reduction potential of the BNN radicals suggested that incorporation into acceptor–donor (A–D) systems would allow for investigation of the role of charge transfer interactions on the electronic structure and properties of neutral open-shell A–D radicals. Two D–A–D radicals were prepared using metal catalyzed coupling and furoxan condensation methodologies which resulted in incorporation of a second donor in the C5 position of the BNN moiety. The radical D1–A–D2 triads, where D1 = thiophene and D2 = thiophene or phenyl, exhibited three intramolecular charge-transfer excitations (λmax = 550, 580 and 1000 nm) that were investigated by variable temperature absorption spectroscopy. Structural characterization of the triads in the solid state by single crystal and powder X-ray diffraction revealed slipped π stacks that arise from intermolecular π– π and D–A interactions, providing pathways for antiferromagnetic (AFM) and ferromagnetic (FM) exchange. While the phenyl substituted triad (Th–BNN–Ph) exhibited antiferromagnetic interactions and a room temperature conductivity of σRT = 10−7 S cm−1, the thienyl substituted derivative (Th–BNN–Th) exhibited short-range FM interactions and increased conductivity (σRT = 10−5 S cm−1), giving rise to an organic semiconductor exhibiting FM exchange. The differences in conductivity and magnetic behavior were rationalized by the degree of slippage dictated by an interplay between π– π and intermolecular D−A interactions. Finally, a series of BNN–D radicals were investigated where the donor ability of D was systematically varied from Eox = 2.30 V vs SCE (benzene) to 0.32 V vs SCE (tetrathiafulvalene). Calculations of the near-infrared charge transfer excitation suggested that the HOMO–SOMO gap could be significantly decreased with increasing donor ability, consistent with charge transfer theory. A subset of the series of BNN–D radicals with D = anisole, benzo[b]thiophene, N-methylindole, N-ethylcarbazole, and N,Ndiphenylaniline were synthesized. Solution state spectroscopic studies of the series by EPR and electronic absorption spectroscopy revealed spin-delocalized structures with extremely low reduction potentials (~0 V vs SCE). The solid state properties of the BNN–D radicals were investigated by magnetometry and room temperature conductivity measurements. Due to primarily steric interactions, weak D–A coupling was observed, with weak intermolecular interactions in the solid state leading to paramagnetic and insulating behaviour. The BNN-(N,N-diphenylaniline) radical structure was characterized by single crystal XRD and found to exist as well isolated radical moieties with extremely weak intermolecular interactions, consistent with magnetometry and conductivity measurements. / Graduate / 0490 / 0794

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This dissertation describes an experimental study on the patterning of thin films and spin valve devices. Initially the change in the magnetisation reversal of ferromagnetic Ni80Fe15Mo5 thin films was investigated as the shape anisotropy was increased using optical lithography to pattern wire arrays. These structures show a progressive increase in coercivity and a transition between single and two-stage reversal with increasing milling depth. A similar patterning technique was applied to unpinned (Ni80Fe20/Cu/Ni80Fe20) pseudo spin valve (PSV) structures in order to enhance the coercivity of one of the ferromagnetic layers. The increased coercivity induced by micropatterning changed the natural similarity of the magnetic layers and the structure exhibited a small spin valve response. These initial measurements were carried out with separate milling and electrical characterisation steps. However, it was decided that it would be ideal to design a technique to do in-situ magnetoresistance measurements during milling. This meant that the samples could be milled and characterised in the same step, leading to a much cleaner and more efficient process. In-situ magnetoresistance measurements were carried out during micropatterning of PSV devices, and the measurements showed the evolution in the electrical response as wire structures were gradually milled through the thickness. Contrary to what was expected, the structures showed a maximum spin valve response when fully milled through. The effect of further increasing the shape anisotropy by reducing the wire width, and changing the material properties in the PSV structure has also been investigated. MR measurements were taken as the temperature was increased from 291K to 493K, and the results show that the patterned PSV structures have a better thermal stability than exchange biased spin valves with an IrMn pinning layer. The experiment was extended to the nanoscale, and the results show that a significant increase in MR is not observed despite the fact that the magnetic configuration tends more towards single domain. This is thought to be due to an increase in the initial resistance of the structures. A small increase in MR was observed as the wire width was decreased from 730 to 470 nm, although the spin valve response is heavily dependent on the gallium dosage density during patterning in the Focused Ion Beam (FIB). Micromagnetic simulations were carried out, which agree with the experimental results and showed the change in the magnetisation reversal from rotation to switching as the dimensions were reduced on the nanoscale.

An experimental investigation of the spins of the 2.14 (Jɪ) and 5.03 (J) Mev levels in B¹¹ has been made using the B¹º(dp)B¹¹ reaction to populate the 5.03 Mev level in B¹¹ and then studying pɣ and pɣɣ angular correlations to determine the values of J and Jɪ . The theoretical analysis of the angular correlation data is based on a method in which the dp reaction mechanism is represented by a relatively small number of experimentally determined parameters and therefore the resulting spin assignments are not open to the usual criticisms of the use of (sometimes doubtful) nuclear reaction theories for the positive determination of nuclear spins. Using the information gained from this experiment and previous experimental information on the statistical distribution of M1 to E2 multipole mixing ratios it was possible to assign an overwhelming statistical probability in favour of the J = ³⁄₂, Jɪ = ½ spin assignment. These spin assignments are in agreement with previous tentative ones and with the theoretical shell model calculations of Cohen and Kurath. The parameters, determined by this experiment, describing the dp reaction are compared with those calculated using stripping theory and are shown to be in disagreement with both the Butler Plane Wave and Distorted Wave Born approximation calculations. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The spin relaxation time, T₁, has been measured at low densities in normal H₂ at 77°K, 196°K, 298°K and 392°K as a function of density using a 96 mHz pulsed N.M.R. spectrometer and a T₁ minimum obtained. The data at 77°K, where only the J=1 rotational state is populated, is fit by the conventional theories, but the results at higher temperatures are not explained by the generalization of the Bloom-Oppenheim⁽⁶⁾ theory. T₁ was also measured as a function of density for a mixture of 54.5% He in at 298°K in an effort to investigate the role of transitions between J states in the relaxation process. The spin relaxation time T₁ was also measured in HD as a function of density in the region of the T₁ minimum at 196°K and 298°K. Since the most recent theory has not been extended to a system of several populated J levels the results are rather inconclusive. T₁ was also measured in CH₄ as a function of density in the region of the T₁ minimum at 196°K and 298°K and information about the rotational coupling constants obtained. From these results it is concluded that measurements of T₁ as a function of density is very useful in testing theories of relaxation and obtaining information on the rotational coupling constants for polyatomic molecules. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

A study of the rates of hindered internal rotation about the C—N bond in N,N-dimethyltrifluoroacetamide and N,N-dimethyldeuteroformamide has been made using the nuclear magnetic resonance spin echo method. A 40 Mc/sec. spin echo spectrometer, which enables measurements to be made of the transverse relaxation time (T₂) from a Carr Purcell sequence of radio-frequency pulses, is described and its performance illustrated. The precision of values obtained is estimated to be of the order of 5%. Equations, derived from a theory developed by Bloom, Reeves and Wells for the dependence of T₂, measured in a Carr Purcell experiment, on pulse intervals in the presence of a suitable exchange process, have been used to extract the kinetic data for the two molecules. The chemical shift, rate constant and natural relaxation time (T2₀) have been calculated from the data by curve fitting procedures using computer techniques, and the experimental verification of the theory has been noted. The accuracy of the rate constants and activation parameters derived is discussed in the light of previous steady state nuclear magnetic resonance studies. The relative merits of the spin echo and steady state methods are examined and the possibility of the occurrence of systematic errors in such studies is investigated. Steady state studies of the proton exchange between methanol and two phenols (meta-cresol and ortho-hydroxyaceto-phenone) and of the ring inversion of N,N-dimethylpiperazine are also presented and the relative merits of the kinetic parameters discussed. / Science, Faculty of / Chemistry, Department of / Graduate

The spin-lattice relaxation time has been studied in normal H₂ as a function of density and temperature in the range 293°K - 700°K. The measurements were made in the region where T₁ α [formula omitted]. The H₂ results have been interpreted using the Bloom-Oppenheim theory in which the transitions between different J states were taken into account. The analysis indicates that the resonant transitions (1,3↔3,1) and quasi-resonant transitions (1,2↔3,0) and (1,4↔3,2) contribute significantly to the relaxation mechanism. The anisotropic inter-molecular potential between the two H₂ molecules which depends on the orientation of both the molecules could be given by quadrupole-quadrupole interaction while the part that depends on the orientation of one of the molecules alone was found to be adequately represented by a Lennard-Jones potential. T₁ was measured in H₂ - He and H₂ - CO₂ mixtures as a function of density and composition in the temperature range 293°K - 700°K. The analysis indicates that the interaction potential for H₂ - He could be adequately described by a Lennard-Jones potential while the dominant interaction for H₂ - CO₂ could be given by quadrupole-quadrupole interaction. There were indications that the dependence of T₁/[formula omitted] in H₂ - He mixture on the percentage of He is non-linear above 150°K. However, this was not found to be the case in H₂ - CO₂ mixtures. T₁ was also measured in CH₄ and CH₄ - He mixture as a function of density and composition in the same temperature range. The data can be fitted by T₁/[formula omitted] = AT⁻ⁿ where n takes the value of 1.5 for pure CH₄ and 0.79 for CH₄ gas infinitely diluted in He. The analysis based on the existing theory for polyatomic gases shows that the intermolecular potential for CH₄ - CH₄ and CH₄ - He could be described by medium range potentials. The results indicate that the dependence of T₁/[formula omitted] on the percentage of He is not linear below 4OO°K. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The longitudinal and transverse nuclear relaxation times, T₁ and T₂, have been measured in normal H₂ gas at 77.5°K in the pressure range 0.05 to 2 atmospheres. In this region T₁ goes through a minimum, and T₂ deviates significantly from a linear dependence on the density. Comparison of the experimental data with existing theory establishes the following results for the J=1 state of orthohydrogen: i. autocorrelation functions of the molecular angular momentum operators are exponential or nearly so, ii. the ratio of the correlation times , Ʈ₁, Ʈ₂, which are associated with operators of the form J₊, and J²₊ respectively, lies within the limits 0.6 ≤ Ʈ₁ / Ʈ₂ ≤ 1, iii. the splitting of the molecular Zeeman levels cannot be neglected as in the original Schwinger theory. T₁ for the proton and deuteron in HD gas and for the deuterons in normal D₂ gas was measured as a function of temperature and pressure in the range 20 to 373°K and 0 to 8 atmospheres. To within experimental error the dependence of T₁ on the density p is linear. In HD below 65°K, when only the J=0 and J=1 states of the molecule are appreciably populated, the temperature dependence of T₁/p is identical for both proton and deuteron, leading to a value of Ʈ₁/Ʈ₂ = 1,07/± 15% for the J=1 state of HD. Above 100°K, T₁/p for the proton is inversely proportional to the temperature, whereas for the deuteron T₁/p is almost temperature independent. The experimental results are interpreted as evidence that in HD gas the process of molecular reorientation is dominated by the anisotropic intermolecular force arising from the separation of the centres of mass and charge of the molecule. In D₂ gas two relaxation times were found, one associated with the S=1 spin state of paradeuterium and the other associated with the S=2 spin state of orthodeuterium. At 40°K (T₁/p)s=₂ appears to go through a minimum; the analogous quantity in H₂ measured by previous workers also goes through a minimum, but at 80°K. This is consistent with interpreting the minimum as a quantum mechanical diffraction effect. The J=2 component of (T₁/p)s=₂ however, does not go through a minimum, which suggests that the intermolecular interactions are significantly different for the J=1 and J=2 states of the molecule. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The cation radical of chlorpromazine in solution was studied in detail by electron spin resonance. The 16 line spectrum was interpreted in terms of a nitrogen atom, two equivalent protons at the first side chain carbon atom, and three almost equivalent protons from the ring system. The relative magnitudes of the splitting constants require many of the spectral lines to be coincident, and the result is the 16 line spectrum observed. Analysis of the splitting constants was done using Huckel molecular orbital calculations, from which it was deduced that the chlorpromazine cation structure is folded about the N-S axis, with an included angle of 104°. The spectral asymmetry observed in sulfuric acid solution was interpreted in terms of random molecular motions causing a fluctuating environment to arise at the nuclear positions. This leads to a modulation effect on the nuclear magnetic moment, and is responsible for linewidth variation. Further broadening due to exchange effects is discussed qualitatively. The asymmetry of the spectra enable the sign of the nitrogen splitting to be estimated -- it was found to be positive. A brief discussion of the electrical properties of chlorpromazine, using HMO calculations, was included, and some discussion of the mechanism of chlorpromazine drug action was also considered for completeness. / Science, Faculty of / Chemistry, Department of / Graduate