Spectroscopic characterization and dynamics of combustionrelevant molecules and reactive intermediates

Nathanael M Kidwell, Purdue University

Abstract

Diminishing fossil fuel reserves and global climate change have spurred significant interest in obtaining accurate experimental and theoretical data to describe the combustion processes of traditional and biomass-derived fuels. One anticipates that key stable and reactive intermediates with unique chemical reactivity and spectral properties are present, necessitating an array of spectroscopic tools for their positive identification. Supersonic jet spectroscopy is a powerful technique typically used to obtain the isomer- and conformation-specific electronic, vibrational, and rotational structure of stable molecules cooled to their vibrational zero-point levels; however, to study reactive intermediates, they must be synthesized in the jet since they are inherently unstable. The present work utilizes a suite of state-of-the-art spectroscopic methods to characterize jet-cooled neutral and free radical isomers that are relevant to combustion, free from the interference of other present species. First, this work presents the electronic and infrared spectral signatures of several important resonance-stabilized radical intermediates possessing the prototypical benzyl (C6H5CH2) electronic structure. An in-depth study was conducted for α-methylbenzyl radical, which is a derivative of the benzyl radical where the methyl group is adjacent to the primary radical site belonging to the extended benzyl π-electronic network. Similarly, the primary radical intermediate, 5-methyl-2-furanylmethyl, was generated from a next-generation biofuel candidate 2,5-dimethylfuran via mass-resolved, discharge-driven means. In order to explore the behavior of these benzyl-like intermediates, resonant two-photon ionization, dispersed fluorescence, and ground- and excited-state infrared spectroscopy in the alkyl stretch region were complementarily employed to describe the state-dependent effects of methyl rocking for the appropriate radicals. Another theme of my graduate work involves solely using infrared spectroscopy to study the photophysics of inden-2-ylmethyl and 1,2,3-trihydronaphthyl radicals which are notable intermediates in forming indene and naphthalene, respectively. Ground- and excited-state infrared spectroscopy were utilized to probe the nonradiative processes occurring in the excited state. Furthermore, the single-conformation infrared spectroscopy in the alkyl stretch/bend regions of indene, indane, 1,2-dihydronaphthalene, 1,4-dihydronaphthalene, and tetralin were investigated. The goal is to build upon a quantitatively accurate first-principles model with predictive capability for alkanes of different types in the alkyl stretch/bend regimes where Fermi resonance often complicates the spectral interpretation. The next part of this work is aimed at understanding the complicated ultraviolet spectroscopy of flexible bichromophores. Here, the bichromophores 1,1-diphenylethane and 1,1-diphenylpropane each possess two ultraviolet chromophores in close proximity to one another. Consequently, this results in their fascinating excited electronic state spectroscopy in which the first two closely-lying excited states are intermingled. Particular emphasis is placed on revealing how the increasing asymmetry from the alkyl chains perturbs the excited state behavior with respect to their symmetric analogue, diphenylmethane. Finally, a novel and powerful method employing chirped-pulse Fourier-transform microwave (CP-FTMW) spectroscopy coupled with a flash pyrolysis (hyperthermal) reactor is described. The exceptional resolution of the method is able to unambiguously identify generated stable and radical systems from selected precursors. In pyrolyzing o-phenylene sulfite, the anti-aromatic reactive intermediate cyclopentadienone was generated, which is ubiquitous in biomass pyrolysis and high-temperature benzene oxidation. Here, the 13C isotopomer species are observed in natural abundance, therefore permitting highly accurate structural determination to report on the nature of anti-aromaticity in cyclopentadienone.

Degree

Ph.D.

Advisors

Zwier, Purdue University.

Subject Area

Physical chemistry

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