Temperature-Dependent Rate Constant for the Reaction of Hydroxyl Radical with 3-Hydroxy-3-methyl-2-butanone

Mechanisms of isomerization and hydration reactions of typical β-diketone at the air-droplet interface

Acetylacetone (AcAc) is a typical class of β-diketones with broad industrial applications due to the property of the keto-enol isomers, but its isomerization and chemical reactions at the air-droplet interface are still unclear. Hence, using combined molecular dynamics and quantum chemistry methods, the heterogeneous chemistry of AcAc at the air-droplet interface was investigated, including the attraction of AcAc isomers by the droplets, the distribution of isomers at the air-droplet interface, and the hydration reactions of isomers at the air-droplet interface. The results reveal that the preferential orientation of two AcAc isomers (keto- and enol-AcAc) to accumulate and accommodate at the acidic air-droplet interface. The isomerization of two AcAc isomers at the acidic air-droplet interface is more favorable than that at the neutral air-droplet interface because the "water bridge" structure is destroyed by H3O+, especially for the isomerization from keto-AcAc to enol-AcAc. At the acidic air-droplet interface, the carbonyl or hydroxyl O-atoms of two AcAc isomers display an energetical preference to hydration. Keto-diol is the dominant products to accumulate at the air-droplet interface, and excessive keto-diol can enter the droplet interior to engage in the oligomerization. The photooxidation reaction of AcAc will increase the acidity of the air-droplet interface, which indirectly facilitate the uptake and formation of more keto-diol. Our results provide an insight into the heterogeneous chemistry of β-diketones and their influence on the environment.

OH Radical and Chlorine Atom Kinetics of Substituted Aromatic Compounds: 4-chlorobenzotrifluoride (p-ClC6H4CF3)

The mechanisms for the OH radical and Cl atom gas-phase reaction kinetics of substituted aromatic compounds remain a topic of atmospheric and combustion chemistry research. 4-Chlorobenzotrifluoride (p-chlorobenzotrifluoride, p-ClC₆H₄CF₃, PCBTF) is a commonly used substituted aromatic volatile organic compound (VOC) in solvent-based coatings. As such, PCBTF is classified as a volatile chemical product (VCP) whose release into the atmosphere potentially impacts air quality. In this study, rate coefficients, k₁, for the OH + PCBTF reaction were measured over the temperature ranges 275-340 and 385-940 K using low-pressure discharge flow-tube reactors coupled with a mass spectrometer detector in the ICARE/CNRS (Orléans, France) laboratory. k₁(298-353 K) was also measured using a relative rate method in the thermally regulated atmospheric simulation chamber (THALAMOS; Douai, France). k₁(T) displayed a non-Arrhenius temperature dependence with a negative temperature dependence between 275 and 385 K given by k₁(275-385 K) = (1.50 ± 0.15) × 10^-14 exp((705 ± 30)/T) cm³ molecule^-1 s^-1, where k₁(298 K) = (1.63 ± 0.03) × 10^-13 cm³ molecule^-1 s^-1 and a positive temperature dependence at elevated temperatures given by k₁(470-950 K) = (5.42 ± 0.40) × 10^-12 exp(-(2507 ± 45) /T) cm³ molecule^-1 s^-1. The present k₁(298 K) results are in reasonable agreement with two previous 296 K (760 Torr, syn. air) relative rate measurements. The rate coefficient for the Cl-atom + PCBTF reaction, k₂, was also measured in THALAMOS using a relative rate technique that yielded k₂(298 K) = (7.8 ± 2) × 10^-16 cm³ molecule^-1 s^-1. As part of this work, the UV and infrared absorption spectra of PCBTF were measured (NOAA; Boulder, CO, USA). On the basis of the UV absorption spectrum, the atmospheric instantaneous UV photolysis lifetime of PCBTF (ground level, midlatitude, Summer) was estimated to be 3-4 days, assuming a unit photolysis quantum yield. The non-Arrhenius behavior of the OH + PCBTF reaction over the temperature range 275 to 950 K is interpreted using a mechanism for the formation of an OH-PCBTF adduct and its thermochemical stability. The results from this study are included in a discussion of the OH radical and Cl atom kinetics of halogen substituted aromatic compounds for which only limited temperature-dependent kinetic data are available.

Kinetic and Mechanistic Investigations of OH-Initiated Atmospheric Degradation of Methyl Butyl Ketone

Methyl butyl ketone (MBK, 2-hexanone) is a common atmospheric oxygenated volatile organic compound (OVOC) owing to broad industrial applications, but its atmospheric oxidation mechanism remains poorly understood. Herein, the detailed mechanisms and kinetic properties of MBK oxidation initiated by OH radicals and subsequent transformation of the resulting intermediates are performed by employing quantum chemical and kinetic modeling methods. The calculations show that H-abstraction at the C4 position of MBK is more favorable than those at the other positions, with the total rate coefficient of k(T) = 4.13 × 10^-14 exp(1576/T) cm³ molecule^-1 s^-1 at 273-400 K. The dominant pathway of unimolecular degradation of the C-centered alkyl radical is 1,2-acyl group migration. For the isomerization of the peroxy radical RO₂, 1,5- and 1,6-H shifts are more favorable than 1,3- and 1,4-H shifts. The multiconformer rate coefficient k_MC-TST of the first H-shift of the RO₂ radical is estimated to be 1.40 × 10^-3 s^-1 at room temperature. Compared to the H-shifts of analogous aliphatic RO₂ radicals, it can be concluded that the carbonyl group enhances the H-shift rates by as much as 2-4 orders of magnitude. The rate coefficients of the RO₂ radical reaction with the HO₂ radical exhibit a weakly negative temperature dependence, and the pseudo-first-order rate constant k'_HO2 = k_HO2[HO₂] is calculated to be 3.32-22.10 × 10^-3 s^-1 at ambient temperature. The bimolecular reaction of the RO₂ radical with NO leads to the formation of 3-oxo-butanal as the main product with the formation concentration of 2.2-7.4 μg/m³ in urban areas. The predicted pseudo-first-order rate constant k'_NO = k_NO[NO] is 2.20-9.98 s^-1 at room temperature. By comparing the k_MC-TST, k'_HO2, and k'_NO, it can be concluded that reaction with NO is the dominant removal pathway for the RO₂ radical formed from the OH-initiated oxidation of MBK. These findings are expected to deepen our understanding of the photochemical oxidation of ketones under realistic atmospheric conditions.

Reaction Rate Coefficient of OH Radicals with d 9-Butanol as a Function of Temperature

d ₉-Butanol or 1-butan-d ₉-ol (D9B) is often used as an OH radical tracer in atmospheric chemistry studies to determine OH exposure, a useful universal metric that describes the extent of OH radical oxidation chemistry. Despite its frequent application, there is only one study that reports the rate coefficient of D9B with OH radicals, k ₁(295 K), which limits its usefulness as an OH tracer for studying processes at temperatures lower or higher than room temperature. In this study, two complementary experimental techniques were used to measure the rate coefficient of D9B with OH radicals, k ₁(T), at temperatures between 240 and 750 K and at pressures within 2-760 Torr. A thermally regulated atmospheric simulation chamber was used to determine k ₁(T) in the temperature range of 263-353 K and at atmospheric pressure using the relative rate method. A low-pressure (2-10 Torr) discharge flow tube reactor coupled with a mass spectrometer was used to measure k ₁(T) at temperatures within 240-750 K, using both the absolute and relative rate methods. The agreement between the two experimental aproaches followed in this study was very good, within 6%, in the overlapping temperature range, and k ₁(295 ± 3 K) was 3.42 ± 0.26 × 10^-12 cm³ molecule^-1 s^-1, where the quoted error is the overall uncertainty of the measurements. The temperature dependence of the rate coefficient is well described by the modified Arrhenius expression, k ₁ = (1.57 ± 0.88) × 10^-14 × (T/293)^4.60±0.4 × exp(1606 ± 164/T) cm³ molecule^-1 s^-1 in the range of 240-750 K, where the quoted error represents the 2σ standard deviation of the fit. The results of the current study enable an accurate estimation of OH exposure in atmospheric simulation experiments and expand the applicability of D9B as an OH radical tracer at temperatures other than room temperature.

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.

Disproportionation Channel of the Self-reaction of Hydroxyl Radical, OH + OH → H2O + O, Revisited

The rate constant of the disproportionation channel 1a of the self-reaction of hydroxyl radicals OH + OH → H₂O + O (1a) was measured at ambient temperature as well as over an extended temperature range to resolve the discrepancy between the IUPAC recommended value (k_1a = 1.48 × 10^-12 cm³ molecule^-1 s^-1, discharge flow system, Bedjanian et al. J. Phys. Chem. A 1999, 103, 7017) and a factor of ca. 1.8 higher value by pulsed laser photolysis (2.7 × 10^-12 cm³ molecule^-1 s^-1, Bahng et al. J. Phys. Chem. A 2007, 111, 3850, and 2.52 × 10^-12 cm³ molecule^-1 s^-1, Altinay et al. J. Phys. Chem. A 2014, 118, 38). To resolve this discrepancy, the rate constant of the title reaction was remeasured in three laboratories using two different experimental techniques, namely, laser-pulsed photolysis-transient UV absorption and fast discharge flow system coupled with mass spectrometry. Two different precursors were used to generate OH radicals in the laser-pulsed photolysis experiments. The experiments confirmed the low value of the rate constant at ambient temperature (k_1a = (1.4 ± 0.2) × 10^-12 cm³ molecule^-1 s^-1 at 295 K) as well as the V-shaped temperature dependence, negative at low temperatures and positive at high temperatures, with a turning point at 427 K: k_1a = 8.38 × 10^-14 × (T/300)^1.99 × exp(855/T) cm³ molecule^-1 s^-1 (220-950 K). Recommended expression over the 220-2384 K temperature range: k_1a = 2.68 × 10^-14 × (T/300)^2.75 × exp(1165/T) cm³ molecule^-1 s^-1 (220-2384 K).

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

The kinetics of the reaction of OH radicals with hydroxyacetone has been investigated as a function of temperature at a total pressure of helium of 2.0-2.1 Torr and over an extended temperature range of T = 250-830 K and as a function of pressure at T = 301 K in the pressure range 1.0-10.4 Torr. The rate constant of the reaction OH + CH₃C(O)CH₂OH → products (1) was measured using both absolute (from the kinetics of OH consumption in excess of hydroxyacetone) and relative rate methods (k₁ = 4.7 × 10^-22 × T^3.25 exp (1410/T) cm³ molecule^-1 s^-1 at T = 250-830 K). The present data combined with selected previous temperature-dependent studies of reaction (1) yield k₁ = 4.4 × 10^-20 × T^2.63 exp (1110/T) cm³ molecule^-1 s^-1, which is recommended from the present work at T = 230-830 K (with conservative uncertainty of 20% at all temperatures). k₁ was found to be independent of the pressure in the range from 1.0 to 10.4 Torr of He at T = 301 K. The present results are compared with previous experimental and theoretical data.