Date of Award
Campus Access Thesis
Master of Science (MS)
Resonance-stabilized radicals are intermediates in hydrocarbon combustion, pyrolysis, and perhaps also circumstellar space. They are important in the formation of PAHs, which partially comprise air pollution and interstellar dust. Herein, characterization of the 1-vinylpropargyl radical, and identification and characterization of the 1-ethynylcyclopenten-1-yl radical, are presented. R2C2PI time-of-flight mass spectrometry was used to measure mass-selected electronic spectra. AIEs were determined from 2-color ion yield scans. Optical-optical hole-burning spectroscopy established the number of isomers present. LIF was used to measure spectra matching those from R2C2PI. SVLE spectroscopy of bands common to LIF and R2C2PI yielded ground state vibrational frequencies and excited state assignments. DFT and CASSCF calculations assisted interpretation of spectra and diagnosis of the newly observed C7H7 radical.
The 1-vinylpropargyl radical exists as two conformers, cis- and trans-, with origin transitions at 21234 cm-1 and 21645 cm-1, respectively. AIEs were determined to be 7.82 eV for trans- and 7.89 eV for cis-, in excellent agreement with theory. While both conformers have Cs ground state geometries, calculations suggest that the excited states are of C1 symmetry. Franck-Condon simulations of origin SVLE spectra agree quite well for a' modes, though intensity of the out-of-plane CCH bend is weaker than predicted. Discrepancies possibly result from Fermi resonance in the ground state and existence of one or more double-minimum potentials for a” modes in the excited state.
An unknown C7H7 isomer, exhibiting an electronic origin band near 21806 cm-1, was first observed in a toluene discharge, in co-existence with benzyl radical. Hole-burning revealed that the spectrum arises from a single conformer with an AIE of 6.93 eV. A Franck-Condon simulation for 1,2,5,6-heptatetraen-4-yl radical is inconsistent with the SVLE spectrum, and a CCSD calculation suggests an AIE lower than 6.9 eV. The same calculation for 1-ethynylcyclopenten-1-yl radical, which contains an embedded trans-1-vinylpropargyl chromophore, agrees quantitatively with experiment, as do several predicted vibrational modes. The carrier of the newly observed spectrum is tentatively identified as 1-ethynylcyclopenten-1-yl radical. It is not included in any current mechanism of benzyl isomerization or decomposition but could be produced by H-addition to ethynylcyclopentene, which has been detected in analysis of benzyl pyrolysis.
Ward, Meredith Lair, "Spectroscopic Investigation and Identification of the 1-Vinylpropargyl Radical and the 1-Ethynylcyclopenten-1-yl Radical" (2019). Graduate Masters Theses. 577.
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