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The work described in this thesis is concerned with cluster-species and related systems, many of which are electron deficient. The term 'electron deficient' is used to describe a polynuclear species in which there are too few valence electrons to allocate a localised 2-centre 2-eleotron bond to every pair of atoms which are within normal covalent bonding distance. The bonding in these systems may be rationalised instead in terms of the relationship between the total number of skeletal electrons provided by the skeletal cluster units, and the total number of skeletal atoms. The aim of this work is to suggest new ways in which bond enthalpy contributions can be allocated to individual 2-centre links in cluster systems. In order to obtain energy terms (E) which reflect changes in bond length, (d), relationships of the form E α d(^-k)(where k=constant; 2<k≤5) are proposed. Such empirical correlations are shown to be appropriate for simple main group systems and are applied in turn to boron hydrides, borane anions, transition metal carbonyls and to complexes containing multiple metal-metal bonds. Similar relationships are used to suggest possible bond orders in some systems. Finally, the extent to which skeletal electron counting methods may be used to rationalise the bonding in boranes, carboranes, transition metal complexes and small clusters, metal π-hydrocarbon complexes and small cyclic hydrocarbons is discussed.

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Structural and equilibrium studies of Meisenheimer complexes formed from activated anisoles and from l-X-3,5-dinitrobenzenes have been carried out. Specific effects of cations on the stabilities of such a complexes were observed and an explanation has been put forward. The results of (^1)H n.m.r. and visible spectroscopy for the interaction of l-chloro-2,6.dinitro-4-X-benzenes (X = Cl, CO(+2)Me, CF(_3)) with sodium methoxide in dimethyl sulphoxide solutions show that initially base addition takes place at an unsubstituted aromatic carbon, forming a O-complex. However the faster addition of base at the unsubstituted ring position is followed by nucleophilic replacement of chloride ion forming the corresponding anisole. Visible spectroscopy was used for the equilibrium and kinetic study of a-complex formation from activated anisoles and methoxide ion in methanol. The values of stoichiometric equilibrium constants so obtained show an increase with base concentration when sodium, potassium or tetra-n-butyl-ammonium methoxide is used as the base, while no such increase is observed with lithium methoxide. It is suggested that the observed increase in stoichiometric equilibrium constants is due to the stabilisation of the complexes by association with the cations present. As a result of this observation it seems more appropriate to use lithium methoxide for the measurements of thermodynamic equilibrium constants. The specific effects of cations are also observed with bivalent barium and calcium ions, in whose presence large increases in the stoichiometric equilibrium constants for methoxide addition to 2_methoxycarbonyl_4, 6_dinitroanisole, 4-methoxycarbonyl-2, 6-dinitroanisole and 2-chloro-4,6-dinitroanisole are found. This effect, attributed to cation association with the Meisenheimer complexes, is demonstrated in the case of the methoxide σ-complex from 2,4,6-trinitro-anisole, by change in visible spectrum in the presence of various cations. However the σ-complex formed from 1,3,5-trinitrobenzene and methoxide ions shows little tendency to form ion associates with cations. It is therefore suggested that when ion pairs are formed the cation is held by a cage effect by the two oxygen atoms of the methoxyl group at the position of addition and by the electronegative substituent at the ortho-position. Structural and equilibrium studies for the formation of Meisenheimer complexes from some l-X-3,5-dinitrobenzenes and methoxide ions were also made. (^1)H n.m.r. results in conjunction with visible spectroscopy indicateinitial addition of base at an aromatic carbon para to the substituents X, however rearrangement occurs so that at equilibrium the isomeric adduct formed from addition of base at a ring carbon para to a nitro-group usually dominates. The measurements of indicator ratios in methanol-dimethyl sulphoxide mixture containing (~10(^-1) sodium methoxide allowed the simultaneous determination of the acidity function and the thermodynamic equilibrium constants, K, for complex formation. Similar measurements of indicator ratios with the less reactive anisoles in methanol-dimethyl sulphoxide mixtures lead to the determination of an alternative J(_M) acidity function and the thermodynamic equilibrium constants for the complex formation from anisoles and methoxide ions. The two scales as well as the stability constants of the adducts formed from l-X-3,5-dinitrobenzenes and similarly activated anisoles are compared.

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No description available.

The chemical reactions of toluene and oxygen in a 50 Hz ac discharge have been studied in a capacitive coupled discharge reactor. The variables investigated were partial pressure of the reactants, reactor pressure, reactor temperature, reactant ratios, applied reactor voltage, discharge current,capacitive current, breakdown voltage, and the phase shift. The thermal reaction was negligible at temperatures below 300 °C. The major products of the reaction between toluene and oxygen were benzaldehyde, o-cresol, benzene, m+p-cresol, phenol and benzyl alcohol. The reaction was controlled by the electric field and the total reactor pressure. The threshold energy for the chemical reaction was found to be about 10 eV. The reaction was found to be of first order with respect to oxygen. The effect of the electric field was correlated by the parameter E/N. The selectivity of benzene was mainly determined by the ratio of toluene to oxygen. Cresol, benzaldehyde, benzyl alcohol and phenol were mainly controlled by the parameter E/N and the partial pressure of oxygen and were all first order with respect to oxygen. A rate expression for bimolecular reactions in cold plasmas was derived and found to agree well with the experimental results. A model was proposed to explain some unusual features of the discharge current and a circuit for measuring the discharge current was described. The energy yield obtained was about 1 kWhr per mole toluene.

Irradiation of molecules by X-rays leads to photo- emission of electons, this forming the basis of XPS or ESCA spectroscopy. The electrons remaining In the molecule experience an effective Increase In nuclear charge accompanying photolonization, and undergo a "relaxation" process. The energy associated with this (the relaxation energy) affects not only the intensity and shape of the experimentally measured peak, but also its position (or binding energy) to a significant extent. By means of well-established quantum mechanical methods. It Is possible to calculate theoretically both binding energies and relaxation energies for core-electron photoionizatlon. Specifically, ab initio LCAO MO SCF calculations within the Hartree-Fock formalism have been performed on a wide variety of organic systems. The relaxation energy as a function of increasing chain- length In a series of linear and bent alkanes has been Investigated, and found to be responsible for the experimentally observed shifts In core binding energies. The valence (2s) Ionized species have also been Investigated, with particular emphasis on line-widths. A study of the simple carbocations has been made, a particular point of Interest being the classical or non-classical descriptions of the bonding in specific cases. To Investigate the effect of extra-molecular contributions to relaxation energies, a series of hydrogen-bonded dimers has been studied as a simple prototype system. Intermolecular contributions to relaxation energies are found to be of the same sign, irrespective of the sign for the shift in core binding energy. The core-like, valence (2s) hole-state species are found to have parallel trends In shifts. A method has been developed for partitioning the total relaxation energy Into contributions from each occupied orbital in the molecule of interest. This procedure Is shown to be both mathematically rigorous, and In accord with chemical Intuition, and Is applied to several series of related molecules. From studies of this nature, a clearer understanding of the relationship between chemical bonding, and the reorganization processes accompanying core-ionization is possible. Finally, as an extension of the Investigation into extra- molecular relaxation energies, and a further example of the use of relaxation energy partitioning, some simple prototype systems for the adsorption of small molecules onto metal surfaces are considered. In the case of CO on Ni, It Is found that a bent conformation Is needed to Interpret the available XPS data.