Observation of the A -- X˜ electronic transition in peroxy radicals using cavity ringdown spectroscopy.

摘要

Alkyl peroxy radicals (RO2) are key intermediates in the low-temperature oxidation of hydrocarbons, a process prevalent in both combustion and atmospheric chemistry. Given that peroxy radicals are involved in many complex reaction mechanisms whose rates can be greatly affected by the structural form of the alkyl peroxy, it is very desirable to have a diagnostic technique which can distinguish between different peroxy radicals (RO2 vs. R 'O2) as well as between different isomers and conformers of the same RO2. Previous spectroscopic studies of the A -- X˜ electronic transition of these alkyl peroxy radicals (located in the near-infrared (NIR)) have shown this to be a diagnostic that is selective among different RO2 and among different isomers and conformers. However, this transition is quite weak and hence requires a sensitive spectroscopic technique to detect it, given the low concentration typical of reactive intermediates. Cavity ringdown spectroscopy (CRDS) overcomes this challenge by providing a longer effective absorption pathlength.; In this research, moderate resolution NIR spectra of methyl peroxy (CH 3O2), ethyl peroxy (C2H5O2), and pentyl peroxy (C5H11O2) have been obtained by room-temperature CRDS. In CH3O2, spectral features such as the A -- X˜ origin ( 000 ) and other vibrational bands were assigned to a single isomer and conformer, while in C2H5O2, we observed origin bands for both predicted stable conformers, trans and gauche. In C5H11O2, we observed a unique electronic spectrum for each of its eight isomers, with multiple conformer origins for each isomer also resolved. Spectral assignments for all three peroxy radicals have been aided by quantum chemistry calculations of the band origin positions, as well as X˜ and A state vibrational frequencies. Combining these data with that previously acquired for propyl (C3H7O2) and butyl (C4H9 O2) peroxy radicals, we are able to develop a systematic understanding of the spectroscopy of this electronic transition, namely what influence a change in the structure of the alkyl peroxy has on the location and structure of its electronic spectrum. Finally, this dissertation concludes with the preliminary moderate resolution NIR spectrum of vinyl peroxy (C 2H3O2), one of the smallest members in a new homologous series of peroxy radicals, namely the alkenyl peroxies.