The purpose of this research was to investigate the kinetics and mechanisms of the photocatalytic degradation of organic substances in the presence of anions (bromide and chloride), using titanium dioxide as a photocatalyst. Tris(hydroxymethyl)aminomethane (Tris) and methanol were the organic substances used as probes. Photocatalytic degradation of the probes produces formaldehyde through reaction with hydroxyl radicals at the surface of the photocatalyst. The product was quantified using a modified Hantzsch reaction and UV-Vis spectroscopy at the fixed wavelength 368 nm. It was found that having bromide present in the reaction mixture resulted in an increase in the rate of formation of formaldehyde from Tris, while it resulted in a decrease from methanol. Bromide on the surface of the photocatalyst reacts with the hydroxyl radicals to form reactive halogen species (RHS). This study proposes that the RHS Br2•- oxidises the probe into a cation radical, which initialises the probe degradation and the subsequent formation of formaldehyde. Conversion from hydroxyl radicals to RHS leads to a greater selectivity in formaldehyde production. Increased selectivity of attack towards electron-rich centres can explain the observed results with the different probes in this study. A linear combination expression of the total production of formaldehyde was developed, through which the X-factor, a ratio of the production of formaldehyde by RHS relative to the production of formaldehyde by hydroxyl radicals, was calculated. However, no realistic values were obtained when calculating the X-factor for different anion concentrations, thus indicating that factors other than competition kinetics affect the degradation. No conclusions could be drawn regarding the effect of chloride on the formation of formaldehyde from Tris and methanol, as the results were ambiguous. / Syftet med denna studie var att undersöka mekanismer och kinetik bakom fotokatalytisk nedbrytning av organiska molekyler i närvaro av anjoner (bromid och klorid), där titandioxid användes som fotokatalysator. Tris(hydroxymetyl)aminometan (Tris) och metanol var de organiska substanserna som studerades. Den fotokatalytiska nedbrytningen av proberna resulterar i formaldehyd via reaktion med hydroxylradikaler på fotokatalysatorns yta. Produkten kan sedan kvantifieras genom en modifierad version av Hantzsch-reaktionen följt av UV-Vis spektroskopi vid en fixerad våglängd på 368 nm. Studien kom fram till att närvaron av bromid i reaktionslösningen resulterade i en ökad produktionshastighet av formaldehyd från Tris, medan det resulterade i en minskning från metanol. Bromid på ytan av fotokatalysatorn reagerar med hydroxylradikaler och bildar reaktiva halogena molekyler (RHS). Denna studie föreslår att RHS:en Br2•- oxiderar proben till en radikalkatjon, som initierar nedbrytningen av proben och efterföljande bildning av formaldehyd. Omvandling från hydroxylradikaler till RHS leder till högre selektivitet av bildning av formaldehyd. Ökad selektivitet av attacker mot elektronrika center kan förklara de observerade resultaten med de olika proberna i denna studie. Ett linjärkombinationsuttryck av den totala produktionen av formaldehyd utvecklades, från vilket X-faktorn, ett förhållande av produktionen av formaldehyd via RHS relativt till produktionen av formaldehyd via hydroxylradikaler, kunde beräknas. Inga realistiska värden erhölls dock vid beräkningen av X-faktorn för olika anjonskoncentrationer, vilket indikerar att andra faktorer än konkurrenskinetik påverkar nedbrytningen. Inga slutsatser kunde dras gällande klorids effekt på bildningen av formaldehyd från Tris och metanol, då resultaten var tvetydiga.

The critically important nature of radical and radical ion mechanisms in biology and chemistry continues to be recognized as our understanding of these unique transient species grows. The work presented herein demonstrates the versatility of kinetic studies for understanding the elementary chemical reactions of radicals and radical ions. Chapter 2 discusses the use of direct ultrafast kinetics techniques for investigation of crucially important enzymatic systems; while Chapter 3 demonstrates the value of indirect competition kinetics techniques for development of synthetic methodologies for commercially valuable classes of compounds. The mechanism of decay for aminyl radical cations has received considerable attention because of their suspected role as intermediates in the oxidation of tertiary amines by monoamine oxygenases and the cytochrome P450 family of enzymes. Radical cations are believed to undergo deprotonation as a key step in catalysis. KIE studies performed by previous researchers indicate N,N-dimethylaniline radical cations deprotonate in the presence of the bases acetate and pyridine. By studying the electrochemical kinetics of the reaction of para substituted N,N-dimethylaniline radical cations with acetate anion, we have produced compelling evidence to the contrary. Rather than deprotonation, acetate reacts with N,N-dimethylaniline radical cation by electron transfer, generating the neutral amine and acetoxyl radical. Transport properties of reactants and solvent polarity changes were investigated and confirmed not to influence the electrochemical behavior forming the basis for our mechanistic hypothesis. To reconcile our conclusion with earlier results, KIEs were reinvestigated electrochemically and by nanosecond laser flash photolysis. Rather than a primary isotope effect (associated with C-H bond cleavage), we believe the observed KIEs are secondary, and can be rationalized on the basis of a quantum effect due to hyperconjugative stabilization in aromatic radical cations during an electron transfer reaction. Product studies performed by constant potential coulometry indicate N,N-dimethylaniline radical cations are catalytic in carboxylate oxidations. Collectively, our results suggest that aminyl radical cation deprotonations may not be as facile as was previously thought, and that in some cases, may not occur at all. Interest in design and synthesis of selenium containing heterocycles stems from their ability to function as antioxidants, anti-virals, anti-inflammatories, and immunomodulators. To establish synthetic feasibility of intramolecular homolytic substitution at selenium for preparation of selenocycles, we set out to determine what factors influence cyclization kinetics. A series of photochemically labile Barton and Kim esters have been syntheisized and employed as radical precursors. The effect of leaving radical stability on kinetics has been investigated through determination of rate constants and activation parameters for intramolecular homolytic substitution of the corresponding radicals via competition experiments. Notable leaving group effects on measured kinetic parameters show more facile reactions for radical precursors with more stable leaving radicals. Moreover, cyclizations to form six-membered (as opposed to five- membered) ring systems exhibited order of magnitude decreases in rate constants for a given leaving radical. Our results are congruent with expectations for radical cyclizations trends for the varied experimental parameters and suggest homolytic substitution affords a convenient means for synthesis of selenocycles. / Ph. D.