High temperature branching ratio of acetaldehyde +OH reaction

The reaction of acetaldehyde (CH3CHO) with hydroxyl radicals (OH) plays an important role in atmospheric and combustion chemistry. The low-temperature chemistry of this reaction has been studied widely in the literature. However, the branching of this reaction at high temperatures is not well known. Aiming to deduce the branching ratio of CH3CHO + OH, measurements were carried out in a shock tube by introducing deuterium into the chemical system. Overall rate coefficients for OH reactions with acetaldehyde (CH3CHO), acetaldehyde-2,2,2-d3 (CD3CHO), acetaldehyde-d4 (CD3CDO) were measured over the temperature range of 950-1300 K and 1.5-3.0 bar. In addition, rate coefficients of OH radicals with acetone (CH3C(O)CH3) and acetone-d6 (CD3C(O)CD3) were measured to deduce the kinetic isotopic effect of H-abstraction reaction at the methyl site. The measured rate coefficients can be represented by the following Arrhenius expressions (cm3/molecule/s): k1 (CH3 CHO + OH) = 1.29 × 10-10 exp (-1996.5 K/T) k2 (CH3 CHO + OH) = 1.06 × 10-10 exp (-2151.9 K/T) k3 (CH3 CDO + OH) = 1.18 × 10-10 exp (-2554.1 K/T) k4 (CH3 COCH3 + OH) = 7.15 × 10-11 exp (-2695.7 K/T) k5 (CD3 COCD3 + OH) = 6.02 × 10-11 exp (-3130.5 K/T) In contrast to the low-temperature chemistry, our results indicate that H-abstraction from the methyl site of acetaldehyde is important at high temperatures and the branching fraction of this channel is ~65%. On per H atom basis, however, H-abstraction from the aldehydic group is faster than that of methyl group even at high temperatures.