Date of Graduation
Eberly College of Arts and Sciences
Reuben H Simoyi
The kinetics and mechanisms of oxidation of selected thiocarbamides (tetra-methylthiourea, trimethylthiourea, phenylthiourea, and 2-aminoethanethiolsulfuric acid) by chlorite in aqueous acidic media are investigated using UV/Vis, NMR, Stopped-flow techniques, and qualitative analysis. The reactions were extremely complex, with reaction dynamics strongly influenced by the pH of the reaction medium and formation of stable intermediates (sulfonic acids). Results revealed that oxidations of substituted thioureas do not always proceed via a stepwise oxidation of the sulfur center. Instead, reactions occurred in two stages: S-oxygenation of the sulfur center to yield the sulfinic acid, which then reacts in the second phase predominantly through an initial hydrolysis to produce a urea-type residue and the sulfoxylate anion. The sulfoxylate anion, a highly reducing species, is then rapidly oxidized to sulfate.;Experimental and numerical studies of local periodic forcing on an excitable Belousov-Zhabotinsky (BZ) medium in a thin gel layer are reported. Rather than the traditional suprathreshold perturbations giving rise to a local oscillatory state, waves were initiated in an excitable system via localized small amplitude variations in light intensity, without crossing into the oscillatory regime of the autonomous system. Initiation of waves in the initially quiescent medium was possible when the frequency of the sinusoidal perturbation was suitably tuned to that of the autonomous system. The region in phase space where wave initiation was possible depended on the parameter values of the perturbation, namely forcing frequency and forcing amplitude, and on the inherent properties of the autonomous system. Resonance patterns are found by relating the period between two waves to the period of the sinusoidal perturbation.;Experimental and theoretical studies of the peroxidase-oxidase (PO) reaction are reviewed. Numerical investigations into the initiation of trigger waves in an oscillatory one-dimensional PO reaction-diffusion system are presented. Trigger waves are initiated in the oscillatory system via localized perturbations in the concentration of one of the variables using the extended BFSO model. The chemical waves traveled with a sharp front and were not able to penetrate barriers to diffusion, which are properties characteristic of trigger waves.
Chigwada-Razunguzwa, Tabitha R., "Dynamics and mechanism studies of nonlinear chemical systems" (2005). Graduate Theses, Dissertations, and Problem Reports. 4141.