Nitroxides in Mechanistic Studies: Ageing of Gold Nanoparticles and Nitroxide Transformation in Acids

The work described in this thesis includes the applications of nitroxide in the mechanistic studies of two very different research topics.

Chapter 2 describes the investigation of ageing of gold nanoparticles (AuNPs) and its mechanistic implications. By utilizing a customized bisnitroxide disulfide, ligand place-exchange reactions of AuNPs were studied by Electron Paramagnetic Resonance (EPR) Spectroscopy and a nitroxide spin label was introduced onto the gold surface in the process. Ageing of thiolate protected AuNPs results in reduced reactivity in the disulfide exchange. By studying the ligand and temperature dependence of this process and the effect of ageing on the maximum extent of exchange, the presence of different binding sites on the surface of AuNPs was proposed.

Chapter 3 describes the mechanistic studies of nitroxide-based polymerization inhibitors in various organic monomers. The presence of nitroxide radicals was utilized to report dissolved oxygen concentration which is essential in the inhibition of polymerization. Oxygen-related broadening was deconvoluted from the EPR spectra of nitroxide radicals using an oximetry fitting method. By adopting this method, the inhibition mechanism and associated chemistry of nitroxides in self-initiated polymerization of acrylic acid was investigated. The synergism of nitroxides with other types of inhibitors was proposed. An unusual decay of nitroxide radicals in acid at high temperature was observed.

Following Chapter 3, Chapter 4 describes the detailed mechanistic investigation of nitroxide decay in acid at high temperature. N-oxoammonium salt, a product of acid-catalyzed disproportionation of nitroxides, was found to decompose at high temperature. Mechanistic investigation of this reaction leads to identification of the important role of hydroxylamine in the mechanism of nitroxide inhibited polymerization of acidic monomers.