A shock tube study of the reaction of the hydroxyl radical with propane

The rate coefficient for the reaction of the hydroxyl radical, OH, with propane has been measured at 1220 K in shock tube experiments, and a value of (1.58 ± 0.24) × 10¹³ cm³/mol s was obtained. This measured value is compared with previous experimental results and a transition-state theory calculation.     

A computational study of the reaction kinetics of methyl radicals with trifluorohalomethanes

Ab initio calculations have been used to characterize the transition states for halogen abstraction by CH₃ in reactions with CF₄, CF₃Cl, CF₃Br, and CF₃I (1–4). Geometries and frequencies were obtained at the HF/6-31G(d) and MP2=full/6-31G(d) levels of theory. Energy barriers were computed via the Gaussian-2 methodology, and the results were employed in transition state theory analyses to obtain the rate constants over 298–2500 K. There is good accord with literature measurements in the approximate temperature range 360–500 K for reactions (2–4), and the computed activation energies are accurate to within ±6 kJ mol⁻¹. Recommended rate constant expressions for use in combustion modeling are k;₁=1.6×10⁻¹⁹ (T/K)^2.41 exp(−13150 K/T), k₂=8.4×10⁻²⁰(T/K)^2.34 exp(−5000 K/T), k₃=4.6×10⁻¹⁹ (T/K)^2.05 exp(−3990 K/T), and k₄=8.3×10⁻¹⁹ (T/K)^2.18 exp(−1870 K/T) cm³ molecule⁻¹ s⁻¹. The results are discussed in the context of flame suppression chemistry. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 179–184, 1998.     

The reaction of acetylene with hydroxyl radicals

The potential energy surface for the reaction between OH and acetylene has been calculated using the RQCISD(T) method and extrapolated to the complete basis-set limit. Rate coefficients were determined for a wide range of temperatures and pressures, based on this surface and the solution of the one-dimensional and two-dimensional master equations. With a small adjustment to the association energy barrier (1.1 kcal/mol), agreement with experiments is good, considering the discrepancies in such data. The rate coefficient for direct hydrogen abstraction is significantly smaller than that commonly used in combustion models. Also in contrast to previous models, ketene + H is found to be the main product at normal combustion conditions. At low temperatures and high pressures, stabilization of the C2H2OH adduct is the dominant process. Rate coefficient expressions for use in modeling are provided.     

A shock tube study of the reactions of the hydroxyl radical with several combustion species

Reactions of the hydroxyl radical, OH, with several organic species of interest in combustion chemistry have been studied near 1200 K and 1 atm in shock tube experiments in which UV absorption was used to monitor the OH concentration. Rate coefficients were measured for the reactions of OH with 2,3-dimethylbutane, isooctane, neooctane, ethylene, propylene, acetylene, formaldehyde, methanol, and ethanol. The values were found to be (in units of 10¹² cm³/mol-s): 21, 22, 18, 2.6, 9.6, 0.28, 12, 5.2, and 5.3. These measured values are compared with previous experimental results and, where appropriate, transition-state theory calculations.     

Interaction between methyl and hydroxyl radicals: a theoretical study

In this paper, we conduct a computational quantum mechanic study of two molecules interaction between methyl (·CH₃) and hydroxyl (·OH) radicals. The molecular exploration has been focused on the possibility of finding non-bonding interactions (formation of complexes by weak-bond interaction among C–O–H atoms) and the potential energy reaction between those two molecules. It shows that the formed complexes presents in slightly lower potential energy than that of the reactants and/or products. The existence of these complexes could proceed to the further interaction, e.g. reaction of the molecules, as those complexes formed a particular configuration.     

Theoretical study of reaction of trifluoromethyl radical with hydroxyl and hydrogen radicals

Ab initio molecular orbital theory and density functional theory calculations have been carried out on the reactions of the trifluoromethyl radical with the hydroxyl and the hydrogen radicals. These reactions are key reactions that underlie a new fire extinguishing mechanism of non-bromine-containing halon replacements. The activation energies calculated by the MP2 and QCISD methods are in good agreement with the experimental values. The B3LYP, as well as MP2 and QCISD, give good results for the calculations of the heats of reactions. The GAUSSIAN-1 and GAUSSIAN-2 theory calculations present the most acxcurate results on both the activation energies and the heats of reactions. The effects of the scaling factors on the activation energies and the heats of reactions are also evaluated. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 277–289, 1998     

First principles study of the reaction of formic and acetic acids with hydroxyl radicals

The oxidation of formic and acetic acids with hydroxyl radicals was studied as a model for the oxidation of larger carboxylic acids using first principles calculations. For formic acid, the CBS-QB3 activation barriers of 14.1 and 12.4 kJ/mol for the acid and for the formyl channel, respectively, are within 3 kJ/mol of benchmark W1U values. Tunneling significantly enhances the rate coefficient for the acid channel and is responsible for the dominance of the acid channel at 298 K. At 298 K, tunneling correction factors of 339 and 2.0 were calculated for the acid and the formyl channel using the small-curvature tunneling method and the CBS-QB3 potential energy surface. The Wigner, Eckart, and zero-curvature tunneling methods severely underestimate the importance of tunneling for the acid channel. The resulting reaction rate coefficient of 0.98 x 10(5) m(3)/(mol x s) at 298 K is within a factor 2-3 of experimental values. For acetic acid, an activation barrier of 11.0 kJ/mol and a tunneling correction factor of 199 were calculated for the acid channel. Two mechanisms compete for hydrogen abstraction at the methyl group, with activation barriers of 11.9 and 12.5 kJ/mol and tunneling correction factors of 9.1 and 4.1 at 298 K. The resulting rate coefficient of 1.2 x 10(5) m(3)/(mol x s) at 298 K and branching ratio of 94% compare well with experimental data.     

Coordinate of the recombination reaction of hydroxyl radicals

The MNDO semiempirical method was used to calculate the hypersurface of the recombination reaction of hydroxyl radicals, the distance between the reactants being varied in the range 0.8–3.2 Å, and the angle of incidence in the range 0–180°. It was found that the reaction takes place in the absence of any appreciable potential barrier. On a time scale of 0–120 fsec, there was observed a double oscillation of the kinetic energy of the colliding particles (25 kJ/mole) and of the potential energy, which was taken as the energy of the saddle point (?159.93 kJ/mole) in the reaction complex during a single collision (10^?13 sec).     

Pulse radiolytic investigation of the reaction of hydroxyl radicals with gentamycin

Hydroxyl radicals were generated radiolytically in N₂O-saturated aqueous solutions of the aminoglycoside antibiotic, gentamycin. Using the pulse radiolysis technique, the rate constant of OH radicals with gentamycin determined was 1.2·10⁹ dm³·mol⁻¹·s⁻¹. Upon^.OH attack a transient species with an absorption maximum at 270 nm is observed which decays by second-order kinetics within the solute concentration range of 3.2·10⁻⁵ to 1·10⁻³mol·dm⁻³. Transient species undergoes transformation to a permanent product absorbing between 260 and 340 nm with maximum absorption at 300 nm. Rate constant of the reaction of bimolecular decay of gentamycin radicals, k (Gen^.+Gen^.) was found to be ≈ 1.4·10⁷ dm³·mol⁻¹·s⁻¹.     

Revisiting the reaction of hydroxyl radicals with vicinal diols in water

The carbonyl products of the reactions of hydroxyl radicals with three vicinal diols (ethane-1,2-diol, propane-1,2-diol and butane-2,3-diol) have been identified and quantified. Hydroxyl radicals were produced by γ-radiolysis of N(2)O-saturated aqueous solutions. The reactions result in the formation of alkoxyl radicals (~15%) followed by β-fragmentation, and α-hydroxyl alkyl radicals that undergo H(2)O elimination. The latter process is part of a radical chain reaction at higher diol concentrations.     

Rate constant for reaction of hydroxyl radicals with bicarbonate ions

The rate constant for reaction of hydroxyl radicals with the bicarbonate ion has been determined to be 8.5 × 10⁶ dm³ mol^-1s^-1. This value was calculated from: the measured rate of formation of the CO^-₃ radical in pulsed electron irradiation of bicarbonate solutions over the pH range 7.0–9.4; the pK for the equilibrium HCO^-₃ ⇌ CO⁼₃ + H⁺; and the rate constant for hydroxyl radicals reacting with the carbonate ion.     

Theoretical studies of the reaction of hydroxyl radical with methyl acetate

The mechanisms and the kinetics of the OH (OD) radicals with methyl acetate CH3C(O)OCH3 are investigated theoretically. The dual-level direct dynamics method is employed in the calculation of the rate constants. The optimized geometries and frequencies and the gradients of the stationary points are calculated at the MP2/6-311G(d,p) level. The energetic information of potential energy surfaces is further refined by the multicoefficient correlation method based on QCISD (MC-QCISD) using the MP2/6-311G(d,p) geometries. Four channels are found for the title reaction. The calculated results reveal that there exists an attractive well (reactant complex) in each entrance H-abstraction channel, that is, the H-abstraction reaction makes a stepwise mechanism. The rate constants are calculated by the canonical variational transition-state theory (CVT) with the interpolated single-point energies (ISPE) approach in the temperature range of 200-1200 K. The small-curvature tunneling effect (SCT) approximation is used to evaluate the transmission coefficient. The calculated rate constants are in good agreement with the experimental ones in the measured temperature range. It is shown that the "out-of-plane hydrogen abstraction" from the methoxy end is the dominant channel at the lower temperatures, and the other two H-abstraction channels should be taken into account with the temperatures increasing. The kinetic isotope effects (KIEs) for the three H-abstraction channels and the total reaction are "inverse", and these theoretically calculated KIEs as a function of temperature are expected to be useful for the future laboratory investigation.     

The rate of reaction of methyl radicals with ozone

The absolute rate constant for the reaction of methyl radicals with ozone has been measured as a function of temperature. Small concentrations of CH₃ were generated by flash photolyzing CH₃NO₂ at 193 nm with an ArF laser. A photoionization mass spectrometer was used to follow the rate of decay of CH₃ at various ozone concentrations. The resulting rate constants could be fit by the expressions over the temperature range of 243–384 K. These rate constants can be modeled by simple transition state theory using reasonable parameters for the activated complex. Use of this rate constant shows that less than 1% of the methyl radicals formed in the stratosphere react with ozone.     

Studies on the reaction of p-hydroxybenzyl alcohol and hydroxyl radicals

The reaction of p-hydroxybenzyl alcohol and hydroxyl radicals generated by the Fenton reaction is studied. The products of the reaction are separated and identified by high-performance liquid chromatography (HPLC)-diode-array detection and HPLC-mass spectrometry. According to the structures of the products, a mechanism of the reaction is proposed.     

A shock tube study of the reaction of the hydroxyl radical with H2, CH4, c-C5H10, and i-C4H10

Reactions of the hydroxyl radical, OH, with several reactants have been studied near 1200 K in shock tube experiments in which UV absorption was used to monitor the OH concentration. The values of the rate coefficients were found to be 2.7 × 10¹², 2.6 × 10¹², 2.8 × 10¹³, and 1.26 × 10¹³ cm³/mol-s for the reactions of OH with hydrogen, methane, cyclopentane, and isobutane, respectively. These measured values are compared with previous experimental results and transition-state theory calculations.     

Ab initio reaction kinetics of hydrogen abstraction from methyl formate by hydrogen, methyl, oxygen, hydroxyl, and hydroperoxy radicals

Combustion of renewable biofuels, including energy-dense biodiesel, is expected to contribute significantly toward meeting future energy demands in the transportation sector. Elucidating detailed reaction mechanisms will be crucial to understanding biodiesel combustion, and hydrogen abstraction reactions are expected to dominate biodiesel combustion during ignition. In this work, we investigate hydrogen abstraction by the radicals H·, CH(3)·, O·, HO(2)·, and OH· from methyl formate, the simplest surrogate for complex biodiesels. We evaluate the H abstraction barrier heights and reaction enthalpies, using multireference correlated wave function methods including size-extensivity corrections and extrapolation to the complete basis set limit. The barrier heights predicted for abstraction by H·, CH(3)·, and O· are in excellent agreement with derived experimental values, with errors ≤1 kcal/mol. We also predict the reaction energetics for forming reactant complexes, transition states, and product complexes for reactions involving HO(2)· and OH·. High-pressure-limit rate constants are computed using transition state theory within the separable-hindered-rotor approximation for torsions and the harmonic oscillator approximation for other vibrational modes. The predicted rate constants differ significantly from those appearing in the latest combustion kinetics models of these reactions.     

Kinetics of the reaction of methyl radical with hydroxyl radical and methanol decomposition

The CH3 + OH bimolecular reaction and the dissociation of methanol are studied theoretically at conditions relevant to combustion chemistry. Kinetics for the CH3 + OH barrierless association reaction and for the H + CH2OH and H + CH3O product channels are determined in the high-pressure limit using variable reaction coordinate transition state theory and multireference electronic structure calculations to evaluate the fragment interaction energies. The CH3 + OH --> 3CH2 + H2O abstraction reaction and the H2 + HCOH and H2 + H2CO product channels feature localized dynamical bottlenecks and are treated using variational transition state theory and QCISD(T) energies extrapolated to the complete basis set limit. The 1CH2 + H2O product channel has two dynamical regimes, featuring both an inner saddle point and an outer barrierless region, and it is shown that a microcanonical two-state model is necessary to properly describe the association rate for this reaction over a broad temperature range. Experimental channel energies for the methanol system are reevaluated using the Active Thermochemical Tables (ATcT) approach. Pressure dependent, phenomenological rate coefficients for the CH3 + OH bimolecular reaction and for methanol decomposition are determined via master equation simulations. The predicted results agree well with experimental results, including those from a companion high-temperature shock tube determination for the decomposition of methanol.     

Rate constants for the reaction of hydrated electrons and hydroxyl radicals with acrylate monomers

The hydrated electron (e⁻_aq) and hydroxyl radical rate constants with 18 acrylate-, methacrylate-, crotonate-, fumarate- and maleate esters are discussed. The constants approach the diffusion-controlled limit. k(e⁻_aq) and k(OH) change in opposite direction; if k(e⁻_aq) is high then k(OH) is small. This tendency is connected with the nucleophilic character of e⁻_aq and the electrophilic character of OH, although the site of attack of e⁻_aq and OH is different: carbonyl versus vinyl group.     

Kinetics of the reaction of OH radicals with t-amyl methyl ether revisited

The kinetics of the reaction of OH radicals with t-amyl methyl ether (TAME) have been reinvestigated using both absolute (flash photolysis resonance fluorescence) and relative rate techniques. Relative rate experiments were conducted at 295 K in 99 kPa (740 torr) of synthetic air using ethyl t-butyl ether, cyclohexane, and di-isopropyl ether as reference compounds. Absolute rate experiments were performed over the temperature range 240–400 K at a total pressure of 4.7 kPa (35 torr) of argon. Rate constant determinations from both techniques are in good agreement and can be represented by k₁=(6.32 ± 0.72) × 10^?12 exp[(?40 ± 70)/T] cm³ molecule^?1 s^?1. Quoted errors represent 2σ from the least squares analysis and do not include any estimate of systematic errors. We show that results from the previous kinetic study of reaction (1) are in error due to the presence of a reactive impurity. Results are discussed in terms of the atmospheric chemistry of TAME. © 1993 John Wiley & Sons, Inc.