No description available.

The main object of the present work has been to calculate nuclear magnetic shieldings for [11]B, [13]C, [14]N (or [15]N), [17]O and [19]F nuclei in a wide variety of organic and inorganic molecules with a view to the understanding of the various electronic factors influencing the observed shieldings. In Chapter 1, current theories of nuclear magnetic shielding are briefly reviewed. A survey of various semi-empirical molecular orbital treatments of magnetic shielding is presented in Chapter 2 with particular emphasis on those employed in the present work, namely Pople's GIAO-MO procedure and the 'Uncoupled Hartree-Fock' method. In Chapters 3-6, Pople's GIAO-MO method has been successfully applied for the calculation of chemical shifts, shielding constants and their anisotropies for B, C, N, 0 and F nuclei in a variety of electronic environments. Reasonable agreement with experimental data is obtained in most cases. The results of comparable calculations based on the 'Uncoupled Hartree-Fock' method are also reported for comparison purposes. Finally, by means of selected examples, it has been demonstrated that a study of contributions to the paramagnetic shielding tensor due to variolas singlet electronic transitions may lead to a better understanding of the various factors governing the observed shieldings.

The main aim of the present work has been to present some theoretical results of nuclear magnetic shieldings for some first row nuclei in a variety of different electronic environments, in order to obtain an improved description of the various factors contributing to nuclear shielding. In Chapter One, several current theories of magnetic shielding are briefly reviewed for purposes of comparison. Chapter Two presents a general survey of various semi-empirical molecular orbital methods of magnetic shielding with particular emphasis on Pople's GIAO-MD approach. This chapter also contains a description of the theory of medium effects on nuclear shielding and a brief introduction to the solvaton model. Chapters Three to Six present 11B, 13C, 14N (or 15N), 17O and 19F chemical shifts, shielding constants and their anisotropies calculated by means of Pople's GIAO-MO method in conjunction with the INDO/S and MINDO/3 parameterization schemes for a wide variety of organic and inorganic molecules. It is demonstrated that the results of the INDO/S calculations are in reasonable agreement with the experimental data, whereas the MINDO/3 parameterization scheme provides quantitatively unreliable results for all of the nuclei considered here. The results obtained are analysed in an attempt to improve our understanding of the relation between magnetic shielding and various features of molecular electronic structure. Finally, the solvaton model is used to explore the role of polar effects on 13C, 14N (or 15N), 17O and 19F shieldings of some polar compounds. This exploration is supplemented by performing some hydrogen-bonding calculations for some model compounds.

The thermodynamics of solution of nonelectrolytes in a number of solvents have been studied theoretically and experimentally. The validity of the two more important theories of solution, i.e. scaled-particle theory (SPT) and Sinanoglu-Reisse-Moura-Ramos theory (SRMR), which have been put forward to account for the solubility of gases and vapours have been investigated. It is shown that neither theory can be used to predict accurately the solubilities of gases and vapours, especially in nonaqueous solvents. Heats of solution of a large, spherical, nonpolar molecule, Me[4]Sn(liquid), have been measured calorimetrically and combined with the known heat of vaporisation to obtain heats of solution of Me[4]Sn(gas) in nonaqueous solvents. Gas chromatographic head space analysis has been used to obtain Raoult's law activity coefficients of Me4Sn in nonaqueous solvents,and thence to obtain DeltaG°[s] , DeltaH°[s] and DeltaS°[s] for solution of Me[4]Sn(gas). The observed values of these quantities have been compared with those calculated by SPT and SRMR theory, and it is shown that neither theory yields values in agreement with experiment. The Menschutkin reaction has been studied kinetically in several solvents at different temperatures, and the activation parameters calculated. Heats of solution of the reactants have been determined and combined with known free energies of solution to yield entropies of solution. Combination with the corresponding activation parameters enables the solvent effect on the transition-state to be determined in terms of G, and S. It is shown that transition-state effects are greater than initial-state effects. Similar experiments have been carried out for the unimolecular reactions of t-butyl halides and it is shown again that in nonaqueous solvents, the solvent effects on the reactants are generally quite small.

The emission spectra of some alkylnaphthalenes containing amino or hydrazine moieties in the side chain exhibit anomalous fluorescence bands. The spectral data are consistent with the proposal that the bands are derived from intramolecular exciplexes. The steric and electronic requirements for intramolecular exciplex formation are discussed on the basis of the dipole moments, wavelengths of fluorescence maxima and, in the case of the hydrazines, the N.M.R. spectra. Irregularities in the rate constants for intermolecular quenching of 9-methylanthracene fluorescence by hydrazines and in the fluorescence spectra of hydrazines are believed to be due to solute association, probably via hydrogen bond formation. A study of the products obtained by irradiation of certain monoacylhydrazines has been made, it is proposed that reaction proceeds by a photoinduced homolysis of the N - N bond. Salts of N,N-diethy1-2-(1-naphthyl)ethylamine with hydrogen halides and alkyl halides have been shown to exhibit exciplex formation in aprotic solvents. The results are interpreted in terms of electron transfer from the halogen anion to the singly excited naphthalene, followed by a rapid elimination of hydrogen halide or alkyl halide. The fluorescence quenching of cyanosubstituted aromatic hydrocarbons has been shown to proceed via exciplex formation when the hydrocarbons possess a high singlet energy. Intramolecular fluorescence quenching of naphthalene by dienes and olefins does not give rise to a fluorescent complex, this has been explained in terms of the character of the quenching groups. Donor-acceptor complexes formed between imides and amines or aromatic hydrocarbons have been shown to give an excited state charge-transfer complex upon excitation. Enhanced complex formation has been observed upon freezing cyclohexane or aceto-nitrile solutions of the compounds. Ester and ether groups have been used to separate two naphthalene moieties. Excitation yields excited state naphthalene complexes even when the linking chain consists of more than three atoms. Similar complexes have been detected in cyclohexane and acetonitrile matrices, it is suggested that excitation of aggregates or crystals formed upon freezing give rise to the new fluorescence bands.

In Part I infinitesimal self-consistent field Hartree-Fock perturbation theory is applied to the calculation of the Fermi contact, electron orbital and spin-dipolar contributions to indirect nuclear spin-spin coupling constants in some small closed-shell molecules from ab initio LCAO-MO SCF wavefunctions. The dependence of the calculated values upon the size of Gaussian basis set employed and upon variations in molecular geometry is examined. The circumstances under which the method may be expected to yield results sufficiently reliable for most chemical purposes are indicated. In Part II the ab initio Unrestricted Hartree-Fock method with single spin annihilation is used in the LCAO-MO approximation with minimal Gaussian basis sets of atomic SCF orbitals to evaluate the isotropic and anisotropic electron-nuclear hyperfine coupling constants in some small doublet state free radicals of current experimental interest. This approach is applied to the identification of new radical species and to the elucidation of their molecular structures. In addition, the method is used to check the experimental assignments of hyperfine coupling constants deduced from ESR spectra and to investigate the reliability of Coulson's equation for the estimation of bond angles in triatomic ABA radicals.

A number of heterocyclic compounds have been used to quench the fluorescence from aromatic hydrocarbons in fluid solution and, in some cases, exciplex emission has been observed. A consideration of solvent effects and the redox potentials of the electron donor and acceptor molecules indicate that the quenching process involves excited state charge transfer interactions. The fluorescence spectra of several monofunctional and bifunctional compounds (aromatic hydrocarbons and amines) have been recorded and fluorescent excimer formation has been observed in some cases. Excimer formation is most efficient when 3 atoms connect the interacting groups. The fluorescence from several aromatic hydrocarbons has been found to be very efficiently quenched by bifunctional compounds such as a,w-diaminoalkanes. When the alkyl chain connecting the terminal amino groups contains less than 5 methylene groups then the quenching process involves both amino groups in the formation of a termolecular complex---in these cases, exciplex emission is relatively inefficient. Exterplex emission has been observed in some cases. The increased quenching efficiency obtained when diamines with 5 or more methylene groups in the connecting chain are used as quenchers is due to multi-collisional quenching rather than exterplex formation. The quenching of the fluorescence from dyes such as Rose Bengal by diaminoalkanes, a,w-dinaphthyl alkanes and related compounds appears to be due to the multi-collisional quenching process rather than exterplex formation. Molecules such as N-n-butyl-N,N-di-(2-naphthylmethyl)-amine and N-phenyl-(2,5-dimethyl) pyrrole, when excited, have more than one possible emitting species depending on the orientation of the interacting groups---one being more polar than the other. The effect of increasing the polarity of the solvent in solutions of these compounds is to stabilize the more polar excited state species and favour emission from this species.

No description available.

Thermoluminescence has been successfully applied to the dating of pottery from archaeological sites and the good agreement obtained between the TL dates and the archaeologically known dates suggested that the basic assumptions concerning the build up and release of trapped charges were correct. In this thesis I report the results of a test programme I carried out on recent volcanic lava of known age: the TL ages were all far too young and hence it was apparent that at least one of the assumptions used in the dating of pottery was invalid when applied to lava. The most obvious differences between pottery and lava dating were studied, such as the effect of crushing such a hard material to obtain fine grains and the influence of the known occurrence of radioactive disequilibrium in recent lavas, but the cause of the low ages was found to be the breakdown of a far more basic assumption. The assumption that electrons in deep traps are capable of remaining in the traps over archaeological time is the basic tenet upon which the whole of TL dating is based. In the case of lava it is shown in this thesis that this does not hold; there was a loss of high temperature TL during storage at room temperature for a few hours. This effect was called 'anomalous fading' as it is in disagreement with thermal untrapping of electrons predicted by kinetic theory and widely accepted in the literature. The third part of this thesis describes the experiments carried out in an attempt to elucidate this phenomenon which was found to be exhibited by a variety of minerals that would normally be considered suitable for the application of TL dating. The same phenomenon has since been reported in the study of the TL of lunar samples and similar effects in other materials are also reported in the final chapter. Two models are suggested which are consistent with the experimental evidence accumulated. They both involve the tunnelling of an electron at low temperatures but differ in their explanation of the thermal dependence observed in experiments at room temperature. The implications of these models with regard to testing for anomalous fading are discussed and it is concluded that, when contemplating the application of TL dating to a new material, a test programme should be carried out on samples of known age and, furthermore, the agreement of the archaeological stability of electrons with that predicted by kinetic analysis of the TL peaks should also be demonstrated. Kinetic analysis carried out on one of the high temperature peaks in limestone agreed with the stability suggested by the geological age of the sample. This confirmed the absence of anomalous fading in this mineral as had been suggested by short-term fading tests. In similar studies on quartz the archaeological stability was found to be greater than that predicted by kinetic analysis using the initial rise method. This discrepency is shown to be due to the inapplicability of the initial rise method to samples in which thermal quenching is occurring. In the past the initial rise has been the most frequently used method of trap depth determination because of its independence of the untrapping kinetics; what has been completely overlooked is that in thermoluminescence the signal will also be affected by any temperature dependence of the luminescence centres. Once this had been allowed for the predicted stability again confirmed the absence of anomalous fading in quartz and hence its suitability for dating.

This thesis describes the mechanisms, applications and limitations of the technique of nuclear quadrupole resonance carried out using double resonance with spin mixing in the laboratory frame. In the first chapter the basic elements of the theory of quadrupole resonance are outlined and the quadrupole Hamiltonian is solved for spin 1 nuclei. The second chapter describes some of the ways in which quadrupole parameters may be used to extract information about the electronic structure of molecules. In chapter III the most common techniques for the measurement of NQR are discussed and some of their advantages and limitations are noted. The next two chapters give a detailed description of the technique of double resonance with spin mixing in the laboratory frame as applied to nitrogen and deuterium nuclei. The events of an experimental cycle are related and the layout of the apparatus is presented. The inherent high sensitivity of the method is attributed to the large population differences given to the quadrupolar nuclei by level crossing processes with the protons. For nitrogen, a simple model where the protons transfer alignment in mutual spin flip transitions involving a single proton and a single nitrogen is shown to predict a lower sensitivity for the technique than that found experimentally The relative sensitivities to the three transitions in nitrogen NQR, as predicted from the simple model, are in qualitative agreement with experiment. In chapter VII the solid effect transitions occuring when a nitrogen is bonded to one, two, or three protons are discussed. The energy of the two proton system, which determines the frequencies of NH₂ solid effect transitions, is solved as a function of applied magnetic field. The fine structure in deuterium pure quadrupole resonance, which occurs when nearby deuterons have nearly degenerate energy levels, is theoretically analysed in chapter VIII. The heights of the subsidiary lines in the fine structure are predicted and the quadrupole lines are assigned from a calculation of the magnetic dipolar interaction between the two nearby deuterons. In chapter IX, the characteristics of the quadrupole spectra from OHD and NHD sites have been qualitatively accounted for by attributing them to the magnetic dipolar interaction between the deuteron and the proton. Since this technique is quite new and the methods of running the apparatus and interpreting the spectra have not yet become well established, a fairly detailed section on operational procedures and the assignment of nitrogen and deuterium quadrupole lines has been included. In the literature there has been some controversy over the assignment of quadrupole constants to the two nitrogen sites in imidazole. In chapter XI the set of lines with e²qQ = 1418 KHz and η = 0.997 is positively assigned to the NH site and the e²qQ = 3253 KHz and η = 0.135 to the N with no covalently bonded protons. In chapter XII the quadrupole resonance technique is applied to the study of hydrogen bonding in amino acids. Three relations of the form e²qQ = (e²qQ)_o - A/R(H...O)³ have been found. (e²qQ)_o represents the e²qQ, of a non-hydrogen bonded site. For O-D...O bonds (e²qQ)_o and A were found to be 328 KHz and 643 KHz-angstrom³. The OD(e²qQ)_o is in good agreement with experimentally determined values. For N±D...O and N±D...Cl bonds the values were 252 KHz and 572 KHz-angstrom³ and 239 KHz and 728 KHz-angstrom³. The average (e²qQ)_o value for a non-hydrogen bonded N⁺D is in excellent agreement with that predicted by a theoretical calculation in the literature. The quadrupole resonance of the water molecule has been carried out in a number of different environments including ice Ih, ice II, clathrate hydrates and molecular complexes. The ice Ih spectrum has been shown to be the sum of two components, one from the HDO molecule and the other from the D₂O molecule. Although this thesis is largely devoted to the quadrupole resonance of ¹⁴N and ²H, a preliminary research project was undertaken to apply the technique to a number of different nuclei with quadrupole transitions in the frequency range around a few MHz. This project was quite successful with quadrupole resonance having been observed in ²³Na, ³⁹K, ¹¹B, ¹⁷O, ²⁶Al and ¹⁰B. This quadrupole data, along with the solutions of the quadrupole Harniltonian for spins ³⁄₂, ⁵⁄₂ and 3 is written up in chapter XIV. Because the processes of laboratory frame double resonance depend on the nuclear spin and the quadrupole frequencies, the observations in chapter V on the mechanisms of the method for nitrogen and deuterium resonance do not necessarily apply to the new nuclei. A comprehensive list of quadrupole data for nitrogen and deuterium is presented in appendices I and II.