Rate constants of long, branched, and unsaturated aldehydes with OH at elevated temperatures

The overall rate constants for the H-abstraction reactions of a series of aldehydes, namely isobutyraldehyde (i-C3H7CHO), n-valeraldehyde (n-C4H9CHO), isovaleraldehyde (i-C4H9CHO), trans-2-pentenal (C2H5CH=CHCHO), trimethylacetaldehyde ((CH3)3CCHO) and benzaldehyde (C6H5CHO), by hydroxyl radicals (OH), were studied behind reflected shock waves at temperatures of 976–1346 K and pressures around 0.6 atm. The OH radicals, produced pyrolytically from tert-butyl hydroperoxide (TBHP), were quantitatively measured by UV absorption using a narrow-linewidth laser near 306.69 nm, targeting the well-characterized R1(5) rovibronic transition of OH in the A-X (0, 0) band. The high sensitivity of this OH diagnostic allowed for very dilute reactant concentrations, which simplified the reaction systems to pseudo-first-order kinetics. For each aldehyde + OH, its overall rate constant was inferred by fitting simulated OH profiles to the measured OH time histories when a detailed reaction mechanism for this aldehyde was available, or directly from exponential fits to the measured OH time-histories (after correcting for a small effect of different TBHP/aldehyde ratios). Both approaches yielded similar results. The resulting reaction rate constants, measured for the first time at temperatures above 1000 K, can be expressed in Arrhenius equations in units of cm3mol−1s−1 as follows: kiC3H7CHO+OH+ = 7.70 × 1013 exp(–1700 K/T) ± 7%, valid over 976 – 1346 K; knC4H9CHO+OH+ = 1.03 × 1014 exp(–1730 K/T) ± 7%, valid over 986 – 1313 K; kiC4H9CHO+OH+ = 1.02 × 1014 exp(–1750 K/T) ± 10%, valid over 976 – 1322 K; k(CH3)3CCHO+OH+ = 7.20 × 1013 exp(–1670 K/T) ± 8%, valid over 979 – 1243 K; kC2H5CHCHCHO+OH+ = 6.93 × 1013 exp(–1340 K/T) ± 8%, valid over 993 – 1285 K; kC6H5CHO+OH+ = 3.10 × 1013 exp(–1380 K/T) ± 8%, valid over 983 – 1333 K.