Photoionization Mass Spectrometric Measurements of Initial Reaction Pathways in Low-Temperature Oxidation of 2,5-Dimethylhexane

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.