jp044080v_si_001.pdf (99.66 kB)
Kinetics of α-Hydroxy-alkylperoxyl Radicals in Oxidation Processes. HO2•-Initiated Oxidation of Ketones/Aldehydes near the Tropopause
journal contribution
posted on 2005-05-19, 00:00 authored by Ive Hermans, Jean-François Müller, Thanh Lam Nguyen, Pierre A. Jacobs, Jozef PeetersA comparative theoretical study is presented on the formation and decomposition of α-hydroxy-alkylperoxyl
radicals, Q(OH)OO• (Q = RR‘C:), important intermediates in the oxidation of several classes of oxygenated
organic compounds in atmospheric chemistry, combustion, and liquid-phase autoxidation of hydrocarbons.
Detailed potential energy surfaces (PESs) were computed for the HOCH2O2• ⇌ HO2• + CH2O reaction and
its analogues for the alkyl-substituted RCH(OH)OO• and R2C(OH)OO• and the cyclic cyclo-C6H10(OH)OO•.
The state-of-the-art ab initio methods G3 and CBS-QB3 and a nearly converged G2M//B3LYP-DFT variant
were found to give quasi-identical results. On the basis of the G2M//B3LYP-DFT PES, the kinetics of the
≈15 kcal/mol endothermal α-hydroxy-alkylperoxyl decompositions and of the reverse HO2• + ketone/aldehyde
reactions were evaluated using multiconformer transition state theory. The excellent agreement with the available
experimental (kinetic) data validates our methodologies. Contrary to current views, HO2• is found to react as
fast with ketones as with aldehydes. The high forward and reverse rates are shown to lead to a fast Q(OH)OO• ⇌ HO2• + carbonyl quasi-equilibrium. The sizable [Q(OH)OO•]/[carbonyl] ratios predicted for
formaldehyde, acetone, and cyclo-hexanone at the low temperatures (below 220 K) of the earth's tropopause
are shown to result in efficient removal of these carbonyls through fast subsequent Q(OH)OO• reactions with
NO and HO2•. IMAGES model calculations indicate that at the tropical tropopause the HO2•-initiated oxidation
of formaldehyde and acetone may account for 30% of the total removal of these major atmospheric carbonyls,
thereby also substantially affecting the hydroxyl and hydroperoxyl radical budgets and contributing to the
production of formic and acetic acids in the upper troposphere and lower stratosphere. On the other hand, an
RRKM−master equation analysis shows that hot α-hydroxy-alkylperoxyls formed by the addition of O2 to
C1-, C2-, and C3-α-hydroxy-alkyl radicals will quasi-uniquely fragment to HO2• plus the carbonyl under all
atmospheric conditions.