Temperature-Dependent Kinetic Study of the Reaction of Hydroxyl Radicals with Hydroxyacetone

Temperature-Dependent Kinetics of the Reactions of the Criegee Intermediate CH2OO with Hydroxyketones

Though there is a growing body of literature on the kinetics of CIs with simple carbonyls, CI reactions with functionalized carbonyls such as hydroxyketones remain unexplored. In this work, the temperature-dependent kinetics of the reactions of CH2OO with two hydroxyketones, hydroxyacetone (AcOH) and 4-hydroxy-2-butanone (4H2B), have been studied using a laser flash photolysis transient absorption spectroscopy technique and complementary quantum chemistry calculations. Bimolecular rate constants were determined from CH2OO loss rates observed under pseudo-first-order conditions across the temperature range 275-335 K. Arrhenius plots were linear and yielded T-dependent bimolecular rate constants: kAcOH(T) = (4.3 ± 1.7) × 10-15 exp[(1630 ± 120)/T] and k4H2B(T) = (3.5 ± 2.6) × 10-15 exp[(1700 ± 200)/T]. Both reactions show negative temperature dependences and overall very similar rate constants. Stationary points on the reaction energy surfaces were characterized using the composite CBS-QB3 method. Transition states were identified for both 1,3-dipolar cycloaddition reactions across the carbonyl and 1,2-insertion/addition at the hydroxyl group. The free-energy barriers for the latter reaction pathways are higher by ∼4-5 kcal mol-1, and their contributions are presumed to be negligible for both AcOH and 4H2B. The cycloaddition reactions are highly exothermic and form cyclic secondary ozonides that are the typical primary products of Criegee intermediate reactions with carbonyl compounds. The reactivity of the hydroxyketones toward CH2OO appears to be similar to that of acetaldehyde, which can be rationalized by consideration of the energies of the frontier molecular orbitals involved in the cycloaddition. The CH2OO + hydroxyketone reactions are likely too slow to be of significance in the atmosphere, except at very low temperatures.

Rate Constant of the Reaction of OH Radicals with HBr over the Temperature Range 235-960 K

The kinetics of the reaction of hydroxyl radicals with HBr, important in atmospheric and combustion chemistry, has been studied in a discharge flow reactor combined with an electron impact ionization quadrupole mass spectrometer in the temperature range 235-960 K. The rate constant of the reaction OH + HBr → H₂O + Br (1) was determined using both a relative rate method (using the reaction of OH with Br₂ as a reference) and absolute measurements, monitoring the kinetics of OH consumption under pseudo-first-order conditions in excess of HBr. The observed U-shaped temperature dependence of k₁ is well represented by the sum of two exponential functions: k₁ = 2.53 × 10^-11 exp(-364/T) + 2.79 × 10^-13 exp(784/T) cm³ molecule^-1 s^-1 (with an estimated conservative uncertainty of 15% at all temperatures). This expression for k₁, recommended for T = 240-960 K, combined with that from previous low temperature studies, k₁ = 1.06 × 10^-11 (T/298)^-0.9 cm³ molecule^-1 s^-1 at T = 23-240 K, allows to describe the temperature behavior of the rate constant over an extended temperature range 23-960 K. The current direct measurements of k₁ at temperatures above 460 K, the only ones to date, provide an experimental dataset for use in combustion and volcanic plume modeling and an experimental basis to test theoretical calculations.