Part I. Interactions between singlet (1Delta)oxygen and nitroxides as seen by electron paramagnetic resonance methods: Quenching, chemically induced electron spin polarization, and applications in oximetry. Part II. Photolysis of dibenzylketones sorbed in MFI zeolite in the presence of spectator molecules: Cage effect, kinetics and external surface sites characterization.

摘要

In Part I, the lowest singlet excited state of the oxygen molecule, O 2(1Delta) is studied by Electron Paramagnetic Resonance (EPR) spectroscopy. In solution, deactivation of O2(1Delta) by nitroxide molecules produces Chemically Induced Electron Spin Polarization (CIDEP). The process is followed in the microsecond time scale by time-resolved EPR. Spin polarization efficiencies per quenching event are found to be dependant on the molecular structure of the nitroxide, although only the nitroxide moiety is responsible for the spin deactivation of O2(1Delta). Where possible, magnetic measurements are coupled with optical methods in order to corroborate results. O2(1Delta) was generated by both photosensitization and photodecomposition of endoperoxide molecules. CIDEP generated in both cases is compared. The decay of the CIDEP signal is controlled by the O2(1Delta) lifetime, providing a sensitive method to measure oxygen concentration. Moreover, the validity of the classical method of oximetry by EPR is extended to oxygen concentrations up to about 10 times larger than those currently used.;In the gas phase, O2(1Delta) generated both by microwave discharge and photosensitization with naphthalene, is detected. Generation efficiencies are precisely determined, as both O2( 1Delta) and the ground state of the oxygen molecule, O2( 3Sigma), can be detected under the same conditions. Line widths of both species are measured by varying temperature and pressure and a comparison in terms of spin relaxation mechanisms is attempted. Both O2( 1Delta) and O2(3Sigma) lines are followed over time. Typical lifetime values of O2(1Delta) are found to be between 100 and 400 ms. Observed quenching rate constants are measured by varying pressure and temperature.;In Part II, the nitroxide spin probing technique is used to characterize the external surface of MFI zeolites by continuous-wave EPR. Strong binding sites are titrated by a direct method and the energy of interaction for a series of dibenzylketones is measured using the Langmuir isotherm. Photolysis in the presence of spectator molecules, such as water or pyridine, displaces dibenzylketones from the internal sites to the external surface.