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Flash Pyrolysis of t-Butyl Hydroperoxide and di-t-Butyl Peroxide: Evidence of Roaming in the Decomposition of Organic Hydroperoxides Paul J. Jones, Blake Riser, and Jingsong Zhang J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b07359 • Publication Date (Web): 28 Sep 2017 Downloaded from http://pubs.acs.org on September 29, 2017
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Flash Pyrolysis of t-Butyl Hydroperoxide and di-t-Butyl Peroxide:
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Evidence of Roaming in the Decomposition of Organic Hydroperoxides
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Paul J. Jones, Blake Riser, and Jingsong Zhang*
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Department of Chemistry, University of California, Riverside, California 92521
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Abstract
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Thermal decomposition of t-butyl hydroperoxide and di-t-butyl peroxide were
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investigated using flash pyrolysis (in a short reaction time of < 100 µs) and vacuum-ultraviolet (λ
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= 118.2 nm) single-photon ionization time-of-flight mass spectrometry (VUV-SPI-TOFMS) at
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temperatures up to 1120 K and quantum computational methods. Acetone and methyl radical
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were detected as the predominant products in the initial decomposition of di-t-butyl peroxide via
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O-O bond fission. In the initial dissociation of t-butyl hydroperoxide, acetone, methyl radical,
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isobutylene, and isobutylene oxide products were identified. The novel detection of the
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unimolecular formation of isobutylene oxide, as supported by the computational study, was
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found to proceed via a roaming hydroxyl radical facilitated by a hydrogen bonded intermediate.
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This new pathway could provide a new class of reactions to consider in the modeling of the low
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temperature oxidation of alkanes.
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* Corresponding author. E-mail:
[email protected]. Tel: 951-827-4197. Also at Air
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Pollution Research Center, University of California, Riverside, CA 92521
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Introduction
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Organic hydroperoxides are well known in the lower temperature regions of flames.1-11
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Early pyrolysis studies demonstrated the expected labile nature of the O-O bond that dominates
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the chemistry of hydroperoxides.1-5 Burgess and Laughlin showed the importance of
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hydroperoxides in combustion with their role in chain breaking of alkanes.4 Since their work, the
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general assumption has been that decomposition occurs almost exclusively through O-O bond
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cleavage leading to the production of hydroxyl radical and an alkoxy radical that rapidly
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undergoes chain cleavage.1-5 Additionally, a second set of pathways found in the work with
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hydroperoxides has been shown to play a role in the formation of cyclic ethers.5-10, 12-14 This can
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be explained by the rearrangement of hydroperoxyalkyl radicals (QOOH) to produce hydroxyl
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radical and the corresponding cyclic ether.6, 8, 10-11, 13-14 This secondary pathway has caused a
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disagreement between the experiment and previously proposed models;8 this discrepancy led to
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the addition of multiple reaction pathways to the kinetic model.6, 8, 10, 13-14 Recent theoretical
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calculations on CH3OOH suggested the formation of weakly interacting fragments via long-
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range/van der Waals interaction in partial decomposition of unimolecular reactants and partial
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association of bimolecular reactants.15 Subsequently, a roaming step was suggested via
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reorientation of fragments from one region of the long range potential to another and a
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decomposition from the long-range interactions yielding final products.15 This provides the
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possibility of an entirely new class of reactions to consider in understanding the decomposition
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of organic hydroperoxides.
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Roaming pathways are a relatively novel unimolecular dissociation mechanism which
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bypass the conventional transition state geometry and involves near dissociation to radical
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intermediates and intramolecular abstraction to directly form stable closed shell products.16
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Experiments and theories have shown that roaming mechanisms are prevalent in the
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decomposition of many species including oxygenated and non-oxygenated alkanes.17-23 This
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suggests that there could be an entirely distinct class of reaction pathways that are not accounted
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for in current models of combustion systems. Most experimental investigations of roaming
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pathways have focused on photodissociation,15-19, 24-26 though recent experimental and theoretical
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work has demonstrated roaming mechanisms in thermal decomposition as well.15,
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Karmarchik et. al. proposed a roaming pathway in the decomposition of β-hydroxyethyl radical
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to produce vinyl and water via a hydrogen-bonded intermediate and then hydrogen-atom
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abstraction by OH radical.19 Thus far, experiments support this mechanism but have not clearly
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established the contribution of this pathway.30-33 The characteristic frustrated decomposition
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produced by a hydrogen-bonded complex between the roaming OH radical and ethene in the
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roaming mechanism proposed by Karmarchik et. al. is of particular interest to this work on
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organic hydroperoxides, in which the OH radical, a strong reagent for H-atom abstraction, can be
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readily released. The theoretical study of the statistics of roaming pathways by Klippenstein et.
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al. further supports this connection with their discovery of the particularly strong attraction at the
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roaming saddle point for the decomposition of methyl hydroperoxide.15, 30-33
20-23, 27-29
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This work focuses on tert-butyl hydroperoxide because it is a useful model system for
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studying the chemistry of organic hydroperoxides. Previous studies have found that the thermal
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decomposition of t-butyl peroxides proceeds by the cleavage of the oxygen-oxygen bond leading
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to production of hydroxyl radical and the t-butoxy radical.1-4 The t-butoxy radical undergoes
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subsequent decomposition into acetone, methyl and other secondary products not produced by
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unimolecular dissociation.1-4 Previous studies on the related di-t-butyl peroxide also showed that
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di-t-butyl peroxide dissociates exclusively via O-O bond cleavage to the t-butoxy radicals
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followed by decomposition of t-butoxy to acetone and methyl radical.34-38 The di-t-butyl
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peroxide does not have the OH radical fragment, and thus the comparison of t-butyl
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hydroperoxide and di-t-butyl peroxide allows for the evaluation of the role that the OH radical
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plays in the decomposition of t-butyl hydroperoxide, including the possible roaming mechanism.
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In the current work, the flash pyrolysis of t-butyl hydroperoxide and di-t-butyl peroxide were
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investigated using flash pyrolysis and vacuum ultraviolet single-photon ionization time-of-flight
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mass spectrometry (VUV-SPI-TOFMS) techniques. The earlier methods of studying peroxides
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were inundated with secondary decompositions and bimolecular reactions due to having
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relatively long residence time (> 0.4 sec), high pressure, or significant wall effects in the reactor,
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or being run in the solvent phase.1-4,34-38 The conditions of the current study provide significant
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improvements in reducing the residence time (60-100 µs) and minimizing the wall effects.39 The
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flash pyrolysis of acetone and isobutylene oxide, the products from the peroxides, were also
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performed to identify any secondary decomposition or photoionization fragmentation of these
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products.
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Experimental and Computational Methods
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The pyrolysis experiments were performed using a flash pyrolysis VUV-TOFMS
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apparatus that has been previously described.40-42 The precursors were diluted to 0.5% in helium
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by bubbling 1 atm of helium through the liquid sample. tert-Butyl hydroperoxide (70% solution
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in water) and di-tert-butyl peroxide (99%) were obtained from Acros Organics and were used
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without further purification. The sample was passed through a pulsed valve and expanded
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sonically (the local gas flow speed equals the local speed of sound) through the pyrolysis micro-
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reactor, with a residence time of ~60 µs in helium. The pyrolysis micro-reactor consisted of a
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SiC tube (heated length 10 mm, 2 mm o.d., 1 mm i.d.). The SiC tube was heated by passing an
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electric current through the tube from two graphite electrodes with the current controlled by a
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Variac transformer. The temperature was measured with a type C thermocouple attached to the
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exterior of the nozzle and has been calibrated to the internal temperature. The conditions of the
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micro-reactor in the current study were similar to those fully characterized in an early study,39
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which minimized the surface reactions and bimolecular reactions. The sample, decomposition
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products, and carrier gas exited the nozzle and underwent supersonic expansion cooling into a
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molecular beam, which then passed into the photoionization region at a pressure of
