| Summary: | 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.
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