Kinetic Study of OH + OH and OD + OD Reactions

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.

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.

A Full-Dimensional Potential Energy Surface and Dynamics of the Multichannel Reaction between H and HO2

In addition to its vital significance in combustion and atmospheric chemistry, the reaction between H' and HO₂ on the ground triplet state represents a prototype with multiple product channels, including H₂ + O₂, OH + OH, O + H₂O, and H + H'O₂. In this work, a full-dimensional accurate potential energy surface (PES) for the title reaction was developed to provide reliable descriptions for all dynamically relevant regions. Using this PES, we adopted the quasi-classical trajectory approach to study the corresponding reaction dynamics, including the reactivity of each product channel and the associated product branching ratio, the product energy distributions, product angular distributions, and associated microscopic mechanisms. For representing distributions of the product energies, such as product translational energy as well as product rotational and vibrational energies, both the traditional histogram and the kernel density estimation (KDE) methods were used and compared. It seems that the features of the resulting distributions in this work are very similar to each other among different methods. The KDE method is suggested for statistics, particularly for those populations with small oscillations in the histogram plot.

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.

Reaction Profiles and Kinetics for Radical-Radical Hydrogen Abstraction via Multireference Coupled Cluster Theory

Radical-radical abstractions in hydrocarbon oxidation chemistry are disproportionation reactions that are generally exothermic with little or no barrier yet are underappreciated and poorly studied. Such challenging multireference electronic structure problems are tackled here using the recently developed state-specific multireference coupled cluster methods Mk-MRCCSD and Mk-MRCCSD(T), as well as the companion perturbation theory Mk-MRPT2 and the established MRCISD, MRCISD+Q, and CASPT2 approaches. Reaction paths are investigated for five prototypes involving radical-radical hydrogen abstraction: H + BeH → H₂+ Be, H + NH₂ → H₂ + NH, CH₃ + C₂H₅ → CH₄ + C₂H₄, H + C₂H₅ → H₂ + C₂H₄, and H + HCO → H₂ + CO. Full configuration interaction (FCI) benchmark computations for the H + BeH, H + NH₂, and H + HCO reactions prove that Mk-MRCCSD(T) provides superior accuracy for the interaction energies in the entrance channel, with mean absolute errors less than 0.3 kcal mol^-1 and percentage deviations less than 10% over the fragment separations of relevance to kinetics. To facilitate combustion studies, energetics for the CH₃ + C₂H₅, H + C₂H₅, and H + HCO reactions were computed at each level of theory with correlation-consistent basis sets (cc-pVXZ, X = T, Q, 5) and extrapolated to the complete basis set (CBS) limit. These CBS energies were coupled with CASPT2 projected vibrational frequencies along a minimum energy path to obtain rate constants for these three reactions. The rigorous Mk-MRCCSD(T)/CBS results demonstrate unequivocally that these three reactions proceed with no barrier in the entrance channel, contrary to some earlier predictions. Mk-MRCCSD(T) also reveals that the economical CASPT2 method performs well for large interfragment separations but may deteriorate substantially at shorter distances.

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

Reactions of hydroxyketones with OH radicals are of importance in atmospheric chemistry and represent a theoretical interest because they proceed through two reaction pathways, formation of a hydrogen-bonded prereactive complex and direct H-atom abstraction. In this work, the kinetics of the reaction of OH radicals with 3-hydroxy-3-methyl-2-butanone (3H3M2B) has been investigated at 2 Torr total pressure of helium over a wide temperature range, T = 278-830 K, using a discharge flow reactor combined with an electron impact ionization quadrupole mass spectrometer. The rate constant of the reaction OH + 3H3M2B → products (1) was determined using both a relative rate method and absolute measurements under pseudo-first-order conditions, monitoring the kinetics of OH consumption in excess of 3H3M2B, k₁= 5.44 × 10^-41T^9.7exp (2820/T) and 1.23 × 10^-11 exp (-970/T) cm³ molecule^-1 s^-1 at T = 278-400 and 400-830 K, respectively (with a total uncertainty of 20% at all temperatures). The rate constant of the reaction OH + Br₂ → HOBr + Br (2) was measured as a part of this study using both absolute and relative rate methods: k₂ = 2.16 × 10^-11 exp (207/T) cm³ molecule^-1 s^-1 at T = 220-950 K (with conservative 10% uncertainty). The kinetic data from the present study are discussed in comparison with previous measurements and theoretical calculations.

Reaction of O(3P) with C3H6: Yield of the Reaction Products as a Function of Temperature

Reaction of oxygen atoms with propene is an important step in combustion processes particularly affecting the profiles of intermediate species and flame speed. The relative importance of different pathways of this multichannel reaction at different temperatures represents significant theoretical interest and is essential for modeling combustion systems. In the present work, we report the first experimental investigation of the products of the O(³P) + C₃H₆ reaction over an extended temperature range (298-905 K). By using a low pressure flow reactor combined with a quadrupole mass spectrometer, the yields of the five reaction products, H atom, CH₃, C₂H₅, CH₂O and OH were determined as a function of temperature between 298 and 905 K: 0.0064 × (T/298)^2.74 exp(765/T), 1.41 × (T/298)^-1.0 exp(-335/T), 0.92 × (T/298)^-1.41 exp(-381/T), 0.17 × (T/298)^0.165 exp(-36/T), and 0.0034 × (T/298)^2.34 exp(788/T), respectively (corresponding to the variation of the respective yields between 298 and 905 K in the ranges 0.08-0.31, 0.46-0.32, 0.26-0.12, 0.15-0.19, and 0.05-011), independent of pressure in the range 1-8 Torr of helium. For the yields of the minor reaction products, H₂ and CH₃CHO the upper limits were determined as 0.2 and 0.05, respectively. These results are compared with the experimental data and theoretical calculations available in the literature.

An accurate multi-channel multi-reference full-dimensional global potential energy surface for the lowest triplet state of H2O2

A full dimensional potential energy surface for the lowest triplet state of H₂O₂ was developed at the MRCI-F12 level.

Determination of the Rate Constant for the OH(X2Π) + OH(X2Π) → O(3P) + H2O Reaction over the Temperature Range 293−373 K

The rate constant for the reaction OH(X2Pi) + OH(X2Pi) --> O(3P) + H2O has been measured over the temperature range 293-373 K and pressure range 2.6-7.8 Torr in both Ne and Ar bath gases. The OH radical was created by 193 nm laser photolysis of N2O to produce O(1D) atoms that reacted rapidly with H2O to produce the OH radical. The OH radical was detected by quantitative time-resolved near-infrared absorption spectroscopy using Lambda-doublet resolved rotational transitions of the first overtone of OH(2,0) near 1.47 microm. The temporal concentration profiles of OH were simulated using a kinetic model, and rate constants were determined by minimizing the sum of the squares of residuals between the experimental profiles and the model calculations. At 293 K the rate constant for the title reaction was found to be (2.7 +/- 0.9) x 10(-12) cm(3) molecule(-1) s(-1), where the uncertainty includes an estimate of both random and systematic errors at the 95% confidence level. The rate constant was measured at 347 and 373 K and found to decrease with increasing temperature.