title: Photoionization mass spectrometric measurements of initial reaction pathways in low-temperature oxidation of 2,5-dimethylhexane.
abstract: Product formation from R + O2 reactions relevant to low-temperature autoignition chemistry was studied for 2,5-dimethylhexane, a symmetrically branched octane isomer, at 550 and 650 K using Cl-atom initiated oxidation and multiplexed photoionization mass spectrometry (MPIMS). Interpretation of time- and photon-energy-resolved mass spectra led to three specific results important to characterizing the initial oxidation steps: (1) quantified isomer-resolved branching ratios for HO2 + alkene channels; (2) 2,2,5,5-tetramethyltetrahydrofuran is formed in substantial yield from addition of O2 to tertiary 2,5-dimethylhex-2-yl followed by isomerization of the resulting ROO adduct to tertiary hydroperoxyalkyl (QOOH) and exhibits a positive dependence on temperature over the range covered leading to a higher flux relative to aggregate cyclic ether yield. The higher relative flux is explained by a 1,5-hydrogen atom shift reaction that converts the initial primary alkyl radical (2,5-dimethylhex-1-yl) to the tertiary alkyl radical 2,5-dimethylhex-2-yl, providing an additional source of tertiary alkyl radicals. Quantum-chemical and master-equation calculations of the unimolecular decomposition of the primary alkyl radical reveal that isomerization to the tertiary alkyl radical is the most favorable pathway, and is favored over O2-addition at 650 K under the conditions herein. The isomerization pathway to tertiary alkyl radicals therefore contributes an additional mechanism to 2,2,5,5-tetramethyltetrahydrofuran formation; (3) carbonyl species (acetone, propanal, and methylpropanal) consistent with β-scission of QOOH radicals were formed in significant yield, indicating unimolecular QOOH decomposition into carbonyl + alkene + OH.
authors.affiliation: Combustion Chemistry Department, Combustion Research Facility, Sandia National Laboratories , Livermore, California 94550-0969, United States.
authors.forename: Brandon
authors.initials: B
authors.lastname: Rotavera
authors.forename: Judit
authors.initials: J
authors.lastname: Zádor
authors.forename: Oliver
authors.initials: O
authors.lastname: Welz
authors.forename: Leonid
authors.initials: L
authors.lastname: Sheps
authors.forename: Adam M
authors.initials: AM
authors.lastname: Scheer
authors.forename: John D
authors.initials: JD
authors.lastname: Savee
authors.forename: Mohamad
authors.initials: M
authors.lastname: Akbar Ali
authors.forename: Taek Soon
authors.initials: TS
authors.lastname: Lee
authors.forename: Blake A
authors.initials: BA
authors.lastname: Simmons
authors.forename: David L
authors.initials: DL
authors.lastname: Osborn
authors.forename: Angela
authors.initials: A
authors.lastname: Violi
authors.forename: Craig A
authors.initials: CA
authors.lastname: Taatjes
mesh_terms: D006586:Hexanes / Q000737:chemistry*; D013058:Mass Spectrometry; D015394:Molecular Structure; D010084:Oxidation-Reduction; D055668:Photochemical Processes; D011789:Quantum Theory; D013696:Temperature
chemical_list: D006586:Hexanes; C000598994:2,5-dimethylhexane