In this thesis, the investigation of the mechanism of chlorine evolution reaction at (i) both freshly reduced and pre-oxidized Pt electrodes; (ii) RuO$\sb2$-TiO$\sb2$ electrodes (DSA); (iii) other noble metals such as Pd, Ir, Ru and Rh by means of computer controlled anodic polarization, potential-relaxation transients and a.c. impedance measurements, will be presented. As described in Chapter V, the influence of co-deposited OH and O species in the surface oxidation process on the extent of chemisorption of the Cl$\sp-$ intermediate in the Cl$\sb2$ evolution reaction has been carefully examined. Experimental results reveal that the reaction rate at reduced Pt surfaces is much greater than that at pre-oxidized ones. This is because the oxide species have a significant effect in blocking the Cl$\sb2$ evolution kinetics. The detectable surface adsorption pseudocapacitance, which is associated with the "overpotential deposited" Cl$\sp-$ intermediate, at freshly reduced Pt and oxide free Pt surfaces in non-aqueous trifluoroacetic acid (TFA) medium supports this conclusion. In Chapter VI, some exploratory experiments at other noble metal (e.g., Pd, Ir, Ru, and Rh) electrodes in non-aqueous TFA solution have shown very interesting interfacial capacitance information, which must be interpreted probably in terms of more than one adsorbed reaction intermediate being present. In Chapter VII, a possible mechanism of recombination kinetically control has been proposed for the Cl$\sb2$ evolution reaction at RuO$\sb2$-TiO$\sb2$ electrodes, based on the observations of their steady-state behaviour, and electrochemical data from potential relaxation and a.c. impedance measurements. Local supersaturation of Cl$\sb2$ in the microstructure of the oxide film appears to make a contribution to the pseudocapacitance behaviour. Some of the results in this work have been published or in press, as listed below: (1) "Surface electrochemistry of the anodic chlorine evolution reaction at Pt influence of co-deposition of the Cl$\sp-$ intermediate", P. Gu and B. E. Conway, J. Chem. Soc. Farada Trans. 86(6) (1990) 923. (2) "Behaviour of the adsorbed Cl$\sp-$ intermediate in anodic chlorine evolution at thin-film RuO$\sb2$ surfaces", P. Gu and B. E. Conway, J. Appl. Electrochem., in press. (3) "Evaluation of Cl$\sp-$ adsorption in anodic chlorine evolution at Pt by means of a.c. impedance and potential relaxation experiments: role of the state of surface oxidation", P. Gu and B. E. Conway, J. Chem. Soc. Faraday Trans., in press.

The work presented in this thesis is primarily focused on the development of pre-fluorescent probes for carbon-centered radicals. Coumarin-TEMPO and quinoline-TEMPO adducts have been prepared by tethering a free radical nitroxide group such as 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) to coumarin and quinoline fluorophores through an ester bond. Some organic derivatives of the quinoline-TEMPO adduct are also presented. Intramolecular quenching of fluorescence due to a spin-exchange mechanism between the singlet excited state of the fluorophore and the doublet ground state of the nitroxide and the restoring of fluorescence due to coupling with a carbon-centered radical are the conceptual bases for this research study. (Abstract shortened by UMI.)

The first part of the thesis is concerned with the problem of temperature dependence of the Tafel slope for various electrochemical processes including the cathodic H$\sb2$ and anodic O$\sb2$ evolution reactions at Au, Pt and Ni in 0.2 N NaOH aqueous solutions at elevated temperatures up to 473 K by means of steady-state polarization experiments. It is found that the commonly assumed representation of the Tafel slope, b, as b = RT/$\beta$F with $\beta$ a constant, often equal to 0.5, is not followed, i.e. $\beta$ is dependent on T. Several examples of this unconventional behaviour of b on temperature are provided in this thesis. The effect of temperature on the surface processes of oxide film formation and reduction in both acid and alkaline aqueous solutions is also examined by means of cyclic voltammetry experiments at temperatures as high as 529 K. In addition to the expected behaviour, such as the higher is the temperature the larger is the rate of reaction, the processes also become more reversible. Experimental examples are provided in this thesis. The second part of the thesis is about the anodic O$\sb2$ evolution reaction at the two "allotropic" forms of lead dioxide, $\alpha$- and $\beta$-PbO$\sb2$ in 1 N HClO$\sb4$ aqueous solutions. The existence of these two forms of PbO$\sb2$ provides a rare opportunity of examining the structural effect in electrocatalysis and kinetics of the O$\sb2$ evolution process. Significant differences between the dimorphs are observed both in the kinetics and the adsorption behaviour of the O and OH intermediates of the reaction. Both potential-relaxation transients, following prior current interruption, and a.c. frequency-response spectroscopy are used in deriving the adsorption behaviour of the reaction intermediates, both H and O/OH species.

This thesis describes a study of the adsorption of anions and neutral organic molecules at gold electrodes using the electrochemical quartz crystal microbalance (EQCM). Differential capacitance, cyclic voltammetry and ring-disk methods were also used to provide further data. In perchlorate solutions (except 0.1 M HClO$\sb4)$ mass responses are flat at potentials positive of the potential of zero charge (E$\sb{\rm pzc})$ in the double layer region of potential. This seems to agree with the accepted view that perchlorate adsorbs weakly at gold. When phosphate is added to 0.1 M KClO$\sb4$ the mass response is seen to increase with increasing potential positive of the E$\sb{\rm pzc}.$ The size of this increase is proportional to the concentration of phosphate species. Differential capacitance and cyclic voltammetry data confirm the adsorption of phosphate species. Adsorption of 4,4$\sp\prime$-bipyridyl was studied because it is one of many compounds that, when adsorbed, promote the electrochemistry of cytochrome c. The presence of 4,4$\sp\prime$-bipyridyl at the electrode surface removes the mass change seen in phosphate buffer and produces a flat mass change over the potential range from $-$0.6 V to 0.6 V. Replacement of adsorbed ions is confirmed by differential capacitance experiments. When the electrode surface is oxidised in the presence of 4,4$\sp\prime$-bipyridyl, no significant mass increase is observed. The mass response also shows that reversal of the place exchange process (where an O species is inserted into the gold lattice upon oxidation) leads to removal of a small amount of adsorbate and an irreversible loss of material from the electrode. A possible mechanism has been proposed which would account for these observations. Finally, EQCM responses show clearly that adsorbed bipyridyl is easily removed from the electrode surface by washing.

No description available.

No description available.

In order to better understand the kinetics of gas phase bimolecular exchange reactions a computer program has been written which allows easy exploration of the time dependent and vibrational level dependent details of this class of reactions. BIMSIM (for BIMolecular exchange reaction SIMulation) is intended as a very flexible "virtual laboratory" which can easily be configured and reconfigured for a wide range of different experiments (e.g., laser pulse or shock tube), different initial conditions (e.g., of temperature, reactant concentration, and molecular environment), for different reactions in this class, and for different levels of approximation. In order to test the validity and demonstrate the use of the program a reaction system was found for which appropriate input data is available and for which suitably detailed analytical calculations have been done. Agreement was found to be excellent. Using BIMSIM, results of chemical interest were obtained for the reaction Br + HCl $\to$ HBr + Cl. It was found that non-equilibrium depletion of the vibrational levels of HCl are as much as a factor of 10 and that they depend on the relative amounts of Br, HCl and inert diluent He, as well as on the temperature, becoming more pronounced around 500 K. Interesting details of the time dependence of the fractional level populations are discussed.

A major application of tandem mass spectrometry is the study of gas-phase ion chemistry. Gas-phase ion chemistry involving ion thermochemistry and ion dissociation characteristics, is one of the foundations of analytical chemistry. A signal (peak) in a mass spectrum is only identified by its mass-to-charge ratio (m/z) and may represent several isomeric ions. Isomeric species having the same element composition can be distinguished by tandem mass spectrometry. An investigation of isomeric (C,H$\sb3,$N,O) $\sp{+-}$ ions provides such an example, where four isomeric ions ( (H$\sb2$NC(H)O) $\sp{+-}$, (H$\sb2$NCOH) $\sp{+-}$, (H$\sb3$CNO) $\sp{+-}$ and (H$\sb2$CNOH) $\sp{+-}$) and their neutral counterparts were identified by a combination of experimental results and a recent high level ab initio molecular orbital theory calculation. Many elusive neutral species of great interest have been accessed by a tandem mass spectrometric technique, neutralization-reionization mass spectrometry (NRMS), which was developed in the last decade. In this thesis, three oxides of carbon, O=C=C=O, C=C=O and O=C=C=C=O, which have been the object of many studies by experimentalists and theoreticians for almost a century, were first observed as stable neutral species in the gas phase by NRMS. In the gas phase, structures of isomeric species having unique behaviours, such as the two cis and trans isomeric 2-buten-dicarboxylic acids, maleic and fumaric acids, can be defined by full investigations of their molecular ions and major fragment ions. The investigations which involved studies of ion thermochemistry and ion dissociation characteristics as well as the use of isotopically labelled compounds provide more information than an early work which used only conventional mass spectrometric techniques.

A liquid chromatographic method for the assay of pindolol and related compounds in the bulk drug was developed. The method resolves six known and several unknown impurities from the drug and each other using a nitrile column, a mobile phase composed of acetonitrile-sodium acetate buffer (35:65), and a UV detector set at 219 nm. Minimum quantifiable amounts of impurities are 0.02% or less relative to the drug. Ten lots of pindolol raw material were evaluated for purity and drug content. Total levels of impurities in these samples, quantitated against pindolol, ranged from about 0.03% to 0.24%. Assay results were within the range of 98.5% to 101.5%.

Many aspects of the electrophoretic properties of end-labeled DNA molecules were studied in free-solutions, in dilute polymer solutions as well as in gels. New theoretical models for the friction coefficient of the hybrid molecules (for all three cases) are presented in this thesis. Our experimental investigation allowed us to validate the theoretical approach. A study of the gel edge electric field gradients in denaturing polyacrylamide gels was also performed and a method to take this gradient into account (in order to obtain reproducible and useful results) is presented. Using that method, the effect of steric trapping in polyacrylamide gels could be investigated (for different electric fields and polyacrylamide concentrations) and the validity of our biased reptation model was experimentally verified. Finally, we demonstrated theoretically that it is possible to build a ratchet system where two molecules having the same charge (DNA and end-labeled DNA fragments) can move in opposite directions when electrophoresed in a zero-integrated pulsed electric field.