The thesis describes experiments on isotope effects on steric and other interactions that may be used to test, for example, the validity of empirical potential functions for non-bonded interactions involving hydrogen. The earlier serendipitous observation of shifts in π → π* UV absorption bands on deuteriation is exploited as a possible probe for examination of the electronic and geometric changes accompanying deuteriation. The effect of deuterium substitution on the free energy differences of conformational equilibria with hydrogen in two possible environments is studied for a series of 2-substituted-1,3-dioxanes and 3-phenyl-3,5,5-trimethylcyclohexanone. The time-averaged NMR chemical shifts of suitable protons in the deuteriated and undeuteriated molecules are used to indicate the positions of the equilibria in several solvents, and model compounds are used to provide shift values from which free energy differences for the equilibrating systems may be calculated with the same systematic errors in both the undeuteriated and deuteriated systems. Possible sources of error are discussed, and the largest is shown to be the standard deviation of measurement of the time-averaged NMR chemical shift differences, leading to errors of 1 to 2 cal/mole in isotope effects from -2 to +16 cal/mole on strain energies ~ 4 kcal/mole. The measured effects were not always of a sign compatible with a reduction of "size" on changing from an undeuteriated to a deuteriated group. Preliminary investigations of systems for which isotope effects on intermolecular proton exchange may be measured, are reported, and isotope effects on the free energy differences between the two hydroxyl groups of partially deuteriated 3,5-di-t-buryl-2,6-dihydroxyacetophenone and of partially deuteriated 3,5-di-t-butyl-2,6-dihydroxybenzaldehyde O-methyloxime of 50 and 20 cal/mole respectively, protium being favoured in the intramolecularly hydrogen bonded group, are measured by the NMR chemical shifts of hydroxyl proton signals. These chemical shift values are used in conjunction with the hydroxyl proton chemical shifts of suitable models to determine the quoted isotopic changes in free energy differences. Isotopic changes in the intensities of UV absorption bands are measured for cis- and trans-3,5-di-t-butylcyclohexane-1,3-diones and interpreted in terms of changes in the free energy difference between keto-enol and diketo tautomers. No such change was detected on deuteriation of acetylacetone, contrary to the results reported by Allred and Thompson. Isotopic shifts in the frequency position of the centroids of UV bands are measured by "difference" spectrometry, a technique whose applicability and accuracy is illustrated by the reproducibility and insensitivity to calibration errors of shifts in the centroids of the IR carbonyl stretching bands between a series of cyclohexanone derivatives measured in the same way. The UV band shifts are interpreted as isotopic changes of differences in the potential energy between the ground state and geometrically different excited states for the enol forms of the above diketones and a series of 4-aminopyridine derivatives. All the observed effects are interpreted in terms of the reduction of zero-point vibrational energy accompanying substitution of deuterium for hydrogen. However the effect of the smaller "size" of deuteriated groups on steric strain energies appears to be accompanied by isotope effects on other contributions, for example those caused by bond polarities, to non-bonded interactions which may enhance or diminish the "size" effect.

No description available.

This Thesis reports an empirical and mathematical appraisal of simple theories of the "ring-current" effect in conjugated systems; it is divided into five parts. Part One gives an extensive historical and critical review of the subject. In Part Two, the empirical utility of simple "ring-current" theories is assessed by confronting their predictions with experimental data on the alternant, condensed, benzenoid hydrocarbons. In Part Three, the mathematical techniques of graph theory are used to investigate the topological aspects of simple "ring-current" calculations and these considerations are exploited in order to rationalise the relative magnitudes of the "ring-current" intensities calculated, via the London-McWeeny method, in arbitrary, conjugated hydrocarbons. Part Four considers the success of "ring-current" approaches, of varying degrees of sophistication, when applied to strongly paramagnetic conjugated-systems. Part Five summarises the general conclusions and assesses the present status of "ring-current" calculations and their possible future-use. For calculations of the relative, intramolecular "ring-current" ¹H-NMR chemical-shifts in the specially parametrised case of the condensed, benzenoid hydrocarbons, even the crudest "ring-current" models may be used with considerable confidence, but calculations on strongly paramagnetic systems are likely to be realistic only if they are based on a wave function that is iteratively self-consistent with respect to resonance integrals and calculated bond-orders. It is concluded that there is not much further headway to be made in the domain of π-electron semi-empirical calculations and that the future probably lies with ab-initio methods, or, at least, with allvalence- electron semi-empirical ones. In the final analysis, therefore, the "ring-current" idea seems destined to remain only a semi-quantitative conceptual-aid in the context in which, by very definition, it belongs namely, that of semi-empirical, π-electron theory.

In this thesis, molecular orbital calculations are used to study a variety of organic reactions, most of which involve molecules containing a large number of atoms. The purpose of the investigation is to elucidate the factors which bring about the stereospecificity found in certain acyclic reactions and to try and relate these factors, if possible, to those that operate in stereospecific pericyclic reactions. <strong>Chapter 1. Molecular Orbital Theory and Organic Reactions.</strong> The application of molecular orbital theory to organic chemistry is a relatively recent phenomenon. One area in particular that has been studied is that class of reactions known as pericyclic reactions. The various concepts and theories that have been used to explain the preferred modes of reactions are reported in this Chapter. The few calculations and simple theories that have been put forward to describe aspects of the complementary field of acyclic reactions are then enunciated. Finally a simple theory based on the use of H&uuml;ckel molecular orbital theory is proposed, which seeks to explain the known confomnational preferences of a large number of acyclic reactions. <strong>Chapter 2. Theoretical Methods.</strong> In this Chapter, a summary of molecular orbital procedures is given, including descriptions of both ab initio and semiempirical methods. The relative advantages and disadvantages of the various approaches are discussed, especially in relation to the type of problems that are involved. The MINDO/2 semiempirical program is described in detail, as is the particular version used (OPTMO) for a large proportion of the work in thesis. Further details of the molecular orbital programs are given in an Appendix. <strong>Chapter 3. Stereospecificity in Organic Reactions.</strong> As a first step in testing the validity of the proposed simple H&uuml;ckel theory of acyclic reactions, ab initio perturbational calculations are performed on allyl and butadiene species containing varying numbers of electrons. However difficulties are encountered in interpreting the results of the sophisticated calculations and relating them to the simple model. A variety of bonding indices are considered and used. It is decided that the only reliable method of analysis must first partition the total energy into uni- and multicentre terms, in order to discover the precise origin(s) of increased stability in a particular intermediate/transition state(T.S.) A means of carrying this out, within the context of the MINDO approximation, is described. With this as the basis for investigation, simple models for the S_N2 and S_E2 reactions are studied. It is found that although certain 2-centre energy terms (i.e. bonding terms between 'non-bonded' atoms) stabilise the preferred geometry-optimised intermediate, as predicted by the simple theory, the decisive terms are several 1-centre energy terms. The origin of these terms cannot be explained by the simple theory. In order to discover how these terms might arise, symmetrical models are examined, with geometries in between those of the two intermediates/T.S.s. These models are distorted by small amounts, towards the two intermediates, and the resulting changes are analysed in terms of perturbation theory. It was observed that inter-molecular orbital mixing is greater for the distortion towards the preferred intermediate and that this increased mixing can account completely for the difference in the energy changes of the two distortions. This differential mixing is also found to persist in the intermediates themselves. In particular, HOMO&pi; - LUMO&sigma;* electron transfer is paramount. Similarly, geometry-optimised models and symmetrical 'intermediate' models are studied for the S_N2&prime; and S_E2&prime; reactions. Again the symmetrical models are distorted towards the final intermediates. Now both HOMO&pi; - LUMO&sigma;* and, to a lesser extent, HOMO&sigma; - LUMO&pi;* electron transfer are of decisive importance in the distortions towards the preferred intermediates. (MB. There were no &pi;* orbitals for the previous models). The qualitative magnitude of HOMO-LUMO, &sigma;/&pi;, mixings can be predicted from the symmetrical 'intermediate' models, simply by con- sideration of the symmetries of the borderline MOs with respect to the symmetry elements maintained during the distortions. These conclusions provide the basis for the study of a variety of stereospecific reactions. Elimination reactions and the backbone rearrangement are examined using symmetrical 'intermediate' models. In all cases HOMO-LUMO, &sigma;/&pi;, considerations lead to the correct prediction for reaction preference, even when the incoming and leaving groups are not identical. Pericyclic reactions are also considered in the same manner and again the allowed modes of reaction are correctly predicted by HOMO/LUMO considerations. The dangers of indiscriminate use of simple theories are shown in the study of the 'S_N2 type' reaction of hydroxide ion with ethylene oxide (to give ethylene glycoxide). The energy partitioning method shows that this is not a simple 'S_N2 type reaction' and hence it comes as no surprise to find that HOMO/LUMO considerations are not applicable here. This in fact underlines the inadequacies of the simple H&uuml;ckel theory for the explanation and prediction of conformational preference. However it might still be applicable in order to differentiate between configurational isomers. This possibility is briefly examined and ideas for further study in this direction are suggested. Finally several points of general interest are discussed, including the relevance of calculations on isolated molecules to chemical reactions in solution and the factors which determine the extent of orbital mixing. A simple perturbational model applicable to many acyclic organic reactions is then advanced, which leads to the same predictions and similar explanations as the more detailed calculations had produced. It also has the advantage of being closely related to the previous treatments of pericyclic reactions. The determining factor in both cases is found to be the total number of mobile electrons involved in the 'decisive' parts of the transition states/intermediates.

The intramolecular excited state interactions of polycyclic aromatic hydrocarbons and halogens have been studied where the aromatic nucleus is separated from the auencher by numerous linking chains. The effects of the interactions on the various aromatic decay parameters are-explained in terms of excited complex formation rather than simple heavy atom theory. Mechanisms for formation of the complexes are discussed and correlated with the different derivatives used in the study. The reactivity of the carbon-halogen bond is found to have a considerable importance on the effects observed, particularly in relation to work at low temperature and studies of the triplet yields. The work, was extended to include the interactions of a variety of amines with aromatic nuclei by means of similar series of derivatives. The geometrical requirements for excited complex formation are discussed in depth and the conclusion is reached that amine exciplexes require very loose qeometries in relation to excited dimers. Finally the excited state interactions of several photosensitisers with a large number of quenchers have been examined. Excited singlet state quenching is shown to be a general phenomenon and the quenching constants are reported for many aromatic hydrocarbon derivatives, amines and anions. The interactions are shown to be dependent on the ionisation potential of the quencher. An important interaction of [beta]-carotene and the excited singlet state of chlorophylla is reported, together with some interactions of photosensitiser triplet states.

No description available.

No description available.

No description available.

No description available.

No description available.