Investigation Into the Radical and Ionic Species That Dictate Chemistry in Our Atmosphere and Space

Investigation into the Radical and Ionic Species that Dictate Chemistry in our Atmosphere and Space Callan Michael Wilcox A thesis in fulfilment of the requirements for the degree of Doctor of Philosophy School of Chemistry Faculty of Science March 2018 i PLEASE TYPE THE UNIVERSITY OF NEW SOUTH WALES Thesis/Dissertation Sheet Surname or Family name: Wilcox First name: Callan Other name/s: Michael Abbreviation for degree as given in the University calendar: School: Chemistry Faculty: Science Title: Investigation into the Radical and Ionic Species that Dictate Chemistry in our Atmosphere and Space Abstract 350 words maximum: (PLEASE TYPE) This thesis embodies two focuses, pertaining to the observation and identification of atmospherically relevant radicals involved in mechanisms with global ramifications on tropospheric air quality, and an investigation into potential radical and ionic carriers hypothesised to exist in the interstellar medium. Isomer specific observations were made of two rotamers the ortho-hydroxycylohexadienyl radical, a radical involved in the atmospheric degradation of benzene. These spectra were untangled by hole burning spectroscopy and assigned by aid of TD-DFT theory. A detailed study of the effects of methyl substitution on the allyl radical ensued, and the spectra of various electronic transitions of the 1,1-dimethylallyl and 1,2-dimethylallyl were reported, in addition to their ionisation potentials. These radicals were confirmed as products formed from the hydrogen addition to isoprene reaction by comparison with their spectra. A series of efforts were made towards elucidating spectroscopic information on the hydroxy isoprenyl class of radicals, generally assumed to be a major intermediate in the formation of secondary organic aerosol from isoprene within the atmosphere. Mass spectra are reported for these radicals, following the discharge of appropriate precursors. Suggestions for the direction of future studies are discussed. Spectra are reported for the resonantly stabilised 9-hydroanthracenyl and 9-deuteroanthracenyl radicals and their cations, as part of our efforts toward resolving the polycyclic aromatic hydrocarbon hypothesis to account for interstellar absorption features such as the diffuse interstellar bands. Spectra of the cations 9-hydroanthracyllium, and 9-deuteroanthracyllium, were recorded by triple resonance depletion spectroscopy. These spectra were assigned by the aid of TD-DFT calculations, and the modelling of 1-D effective potentials. While not matching any diffuse interstellar features, their spectroscopy is interesting from a fundamental perspective. 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Date …………………………………………….............. iii Abstract This thesis is split into two parts, the first part of which contains an investigation into radical species that have important roles in both urban and pristine atmospheric mechanisms, representing the primary focus of this thesis. The second part of this thesis is concerned with the identification and spectroscopy of radicals and cations that are candidates for absorption features observed in the interstellar medium. Spectroscopic methods are utilised in order to identify and characterise the electronic and vibrational transitions of these molecules. Most extensively used throughout this work is the resonance enhanced multiphoton ionisation (REMPI) technique. Various REMPI schemes are employed to measure the mass, electronic transitions, excited state lifetimes and ionisation energies of various radicals and cations. To aid in assignment of these spectra, we rely on a wide variety of theoretical calculations, including ab initio electronic structure theory, density functional theory and a variety of effective potentials. Within urban environments, formation of aerosol and consequently pho- tochemical smog owes chiefly to the oxidation of aromatics, in particular benzene, initiated by the addition of a hydroxyl radical, forming hydroxycylohexadienyl radicals. Two rotamers of the ortho-hydroxycyclohexadienyl isomer are identified following a discharge of phenol and water vapor. The complex D1 ← D0 spectrum is separated into two spectra, owing to each rotamer, by hole burning spectroscopy. −1 The electronic origins of the anti and syn rotamers D1 states lie at 19182 cm and 19200 cm−1 respectively. The rich vibrational structure of each isomer is assigned by comparison with time-dependent density functional theory computations. Duschin- iv sky matrices are created to map the change in character of modes upon excitation, and are used assign the symmetry of each rotamer in the D1 state. The dominant biogenic volatile organic compound released into the atmosphere is isoprene. The mechanism for its atmospheric loss is also initiated by hydroxyl addition to one of the two double bonds, forming a resonantly stabilised hydroxy isoprenyl radicals with an allylic resonance. As such, a study was undertaken to understand the effects of methyl substitution on allyl, with a goal to understand- ing the spectroscopy of the hydroxy isoprenyl radicals. Following calibration studies on the Rydberg transitions of allyl and 2-methyl allyl, we report various electronic transitions of the 1,1-dimethylallyl and 1,2-dimethylallyl radicals. The D1 and D2 origins of the 1,1-dimethylallyl radical are observed at 24058 cm−1 and 38329 cm−1 respectively. The vibrational structure of the D1 state is dominated by methyl ro- tor transitions, coupling to the electronic and vibronic modes of the radical, which are assigned with aid from computational algorithms. Transitions to the D2 and −1 D3 states of the 1,2-dimethylallyl radical are also observed, at 38501.2 cm and ∼39600 cm−1 respectively. Finally, hydrogen atom addition to isoprene within a discharge environment was studied. The spectra in the visible and UV were identified to contain signatures of both the 1,1- and 1,2-dimethylallyl radicals, in addition to other peaks, which were tentatively assigned as owing to inner addition products. We report considerable efforts towards elucidating spectroscopic

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