Low-Temperature Combustion Chemistry of <i>n-</i>Butanol: Principal Oxidation Pathways of Hydroxybutyl Radicals
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Abstract
Reactions
of hydroxybutyl radicals with O<sub>2</sub> were investigated by a
combination of quantum-chemical calculations and experimental measurements
of product formation. In pulsed-photolytic Cl-initiated oxidation
of <i>n</i>-butanol, the time-resolved and isomer-specific
product concentrations were probed using multiplexed tunable synchrotron
photoionization mass spectrometry (MPIMS). The interpretation of the
experimental data is underpinned by potential energy surfaces for
the reactions of O<sub>2</sub> with the four hydroxybutyl isomers
(1-hydroxybut-1-yl, 1-hydroxybut-2-yl, 4-hydroxybut-2-yl, and 4-hydroxybut-1-yl)
calculated at the CBS-QB3 and RQCISD(T)/cc-pV∞Z//B3LYP/6-311++G(d,p) levels of theory. The observed
product yields display substantial temperature dependence, arising
from a competition among three fundamental pathways: (1) stabilization
of hydroxybutylperoxy radicals, (2) bimolecular product formation
in the hydroxybutyl + O<sub>2</sub> reactions, and (3) decomposition
of hydroxybutyl radicals. The 1-hydroxybut-1-yl + O<sub>2</sub> reaction
is dominated by direct HO<sub>2</sub> elimination from the corresponding
peroxy radical forming butanal as the stable coproduct. The chemistry
of the other three hydroxybutylperoxy radical isomers mainly proceeds
via alcohol-specific internal H-atom abstractions involving the H
atom from either the −OH group or from the carbon attached
to the −OH group. We observe evidence of the recently reported
water elimination pathway (Welz et al. <i>J. Phys. Chem. Lett.</i> <b>2013</b>, <i>4</i> (3), 350–354) from
the 4-hydroxybut-2-yl + O<sub>2</sub> reaction, supporting its importance
in γ-hydroxyalkyl + O<sub>2</sub> reactions. Experiments using
the 1,1-<i>d</i><sub>2</sub> and 4,4,4-<i>d</i><sub>3</sub> isotopologues of <i>n</i>-butanol suggest
the presence of yet unexplored pathways to acetaldehyde
