Flow reactor studies and kinetic modeling of the H2/O2 reaction

Profile measurements of the H₂/O₂ reaction have been obtained using a variable pressure flow reactor over pressure and temperature ranges of 0.3–15.7 atm and 850–1040 K, respectively. These data span the explosion limit behavior of the system and place significant emphasis on HO₂ and H₂O₂ kinetics. The explosion limits of dilute H₂/O₂/N₂ mixtures extend to higher pressures and temperatures than those previously observed for undiluted H₂/O₂ mixtures. In addition, the explosion limit data exhibit a marked transition to an extended second limit which runs parallel to the second limit criteria calculated by assuming HO₂ formation to be terminating. The experimental data and modeling results show that the extended second limit remains an important boundary in H₂/O₂ kinetics. Near this limit, small increases in pressure can result in more than a two order of magnitude reduction in reaction rate. At conditions above the extended second limit, the reaction is characterized by an overall activation energy much higher than in the chain explosive regime. The overall data set, consisting primarily of experimentally measured profiles of H₂, O₂, H₂O, and temperature, further expand the data base used for comprehensive mechanism development for the H₂/O₂ and CO/H₂O/O₂ systems. Several rate constants recommended in an earlier reaction mechanism have been modified using recently published rate constant data for H + O₂ (+ N₂) = HO₂ (+ N₂), HO₂ + OH = H₂O + O₂, and HO₂ + HO₂ = H₂O₂ + O₂. When these new rate constants are incorporated into the reaction mechanism, model predictions are in very good agreement with the experimental data. ©1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 113–125, 1999     

UV absorption spectra of HO2 and CH3O2 radicals and the kinetics of their mutual reactions at 298 K

We report results of a flash photolysis study of the UV, spectra of HO₂ and CH₃O₂ radicals, obtained by using a calibration technique based on the reaction Cl+NO→NOCl. We also report preliminary results from our study of the kinetics of the reaction CH₃O₂+HO₂→products at room temperature and near atmospheric pressure. Our results are consistent with the only previous direct determination of the rate constant of the second reaction: k₁ = (6.4 ± 1.0) × 10⁻¹²cm₃ molecule⁻ s⁻¹. From the same study we derive rate constants for the self-reaction of HO₂ and CH₃O₂ radicals, which agree with recommended values.     

The temperature dependence of the HO2 + HO2 reaction

The temperature dependence of the rate constant for the reaction HO₂ + HO₂ → H₂O₂ + O₂ (2k₁) has been determined using flash photolysis techniques, over the temperature range 298–510 K, in a nitrogen diluent at a total pressure of 700 Torr. The overall second order state constant is given by k₁ = (4.14 ± 1.15) × 10^?13 exp[(630 ± 115)/T] cm³ molecule^?1 s^?1, where the quoted errors refer to one standard deviation. This result is compared with previous findings and the negative activation energy is shown to be consistent with the observation that the rate constant is pressure dependent at 700 Torr.     

Theoretical study on mechanisms and kinetics of NCCO + O2 reaction

Mechanisms and kinetics of the NCCO + O2 reaction have been investigated using the extrapolated full coupled cluster theory with the complete basis set limit (FCC/CBS) and multichannel RRKM theory. Energetically, the most favorable reaction route involves the barrierless addition of the oxygen atom to one of the carbon atoms of NCCO and the subsequent isomerization-decomposition via the four-center intermediate and transition state, leading to the final products NCO and CO2. At 298 K, the calculated overall rate constant is strongly pressure-dependent, which is in good agreement with the available experimental values. It is predicted that the high-pressure limit rate constants exhibit negative temperature dependence below 350 K. The dominant products are NCO and CO2 at low pressures (ca. <10 Torr) and the NCCO(O2) radical at higher pressures, respectively.     

Quasi-hydrostatic X-ray powder diffraction study of the low- and high-pressure phases of CaWO4 up to 28 GPa

We have studied CaWO₄ under compression using Ne as pressure-transmitting medium at room temperature by means of synchrotron X-ray powder diffraction. We have found that CaWO₄ beyond 8.8 GPa transforms from its low-pressure tetragonal structure (scheelite) into a monoclinic structure (fergusonite). The high-pressure phase remains stable up to 28 GPa and the low-pressure phase is totally recovered after full decompression. The pressure dependence of the unit-cell parameters, as well as the pressure–volume equation of state, has been determined for both phases. Compared with previous studies, we found in our quasi-hydrostatic experiments a different behavior for the unit-cell parameters of the fergusonite phase and a different transition pressure. These facts suggest that deviatoric stresses influence on the high-pressure structural behavior of CaWO₄ as previously found in related compounds. The reported experiments also provide information on the pressure dependence of interatomic bond distances, shedding light on the transition mechanisms.     

Thermophysical properties of azomethine ether main-chain liquid crystalline polymers at pressures to 200 MPa

This article reports on an experimental investigation of the equation of state and the transition behavior of main-chain thermotropic liquid crystalline polymers over a wide temperature range, and at pressures to 200 MPa. The materials studied were a series of azomethine ether polymers. A varying number n (= 4, 7, 8, 9, 10 and 11) of methylene spacer units in the backbone provided systematic variation of the structure. Experimental techniques used included high-pressure dilatometry (PVT measurements) to 200 MPa, high-pressure differential thermal analysis, also to 200 MPa, and conventional (atmospheric-pressure) differential scanning calorimetry (DSC). The equation of state of the materials can be well represented by the Tait equation in distinct regions, separated by a glass transition, T_g(P), a first-order transition to a nematic state, T_k-n(P), and a first-order transition to an isotropic melt state T_c(P). The atmospheric pressure values of T_k-n and T_c decreased with increasing number of spacer units and showed a clear odd-even effect. T_g and T_k-n both increased with pressure. The pressure dependence of T_c could not be observed due to the onset of degradation in the same temperature region. On isobaric cooling at 3°C/min, the crystallization from the nematic state occurred a few tens of degrees below T_k-n. This supercooling was independent of pressure for some materials, while for others it increased with increasing pressure. The values of the enthalpy and entropy associated with the first-order transition into the nematic state were lower than those of typical isotropic polymers at their melting transitions. The transition enthalpy did not have any systematic variation with increasing number of spacer units. Values of the transition enthalpy calculated from the Ciapeyron equation did not always agree with the values measured by DSC. This may be due to the two-phase nature of the low-temperature state. At the transition to the isotropic state, the transition enthalpy at P = 0 decreased with n and showed an odd-even effect. © 1994 John Wiley & Sons, Inc.     

Kinetic measurements of the gas phase HO2+CH3O2 cross-disproportionation reaction at 298 K

Flash photolysis kinetic absorption spectroscopy was used to investigate the gas phase reaction between hydroperoxy (HO₂) and methylperoxy (CH₃O₂) radicals at 298 K. Due to the large difference between the self-reactivities of the two radicals, first- or second-order kinetic conditions could not be maintained for either species. Thus, the rate constant for the cross reaction was determined from computer-modeled fits of the radical absorption decay curves, at wavelengths between 215 and 280 nm. This procedure yielded k = 2.9 × 10⁻¹² cm³ molecule⁻¹ s⁻¹ independent of total pressure (using N₂) between 25 and 600 Torr, and of the partial pressure of water vapor (up to 11.6 Torr). There was also no effect of water vapor on the rate constant for the self-reaction of methylperoxy radicals.     

Modified transition state theory and negative apparent activation energies of simple metathesis reactions: application to the reaction CH3 + HBr --> CH4 + Br

A modified transition state theory (MTST) has been developed for gas-phase reactions with "negative barriers". The theory was applied to the reactions CH3 + HBr(DBr) --> CH4(CH3D) + Br (1a, 1b), which exhibit negative temperature dependences. Accurate ab initio calculations performed with coupled cluster theory extrapolated to the complete basis set limit revealed a transition state located at -2.3 kJ mol(-1) relative to the ground state of the reactants (in reaction 1a), as well as a shallow bound complex. The negative temperature dependence, the absolute values of the rate constant, and the isotope substitution effect are reproduced with good accuracy (10%), without any adjustment or fitting parameters. Analytical expressions are presented for MTST including angular momentum conservation, centrifugal barriers and tunneling. This analysis uses information about the possibly loose entrance barrier and the transition state but does not invoke a statistical intermediate complex.     

Direct kinetics study of the temperature dependence of the CH2O branching channel for the CH3O2 + HO2 reaction

A direct kinetics study of the temperature dependence of the CH₂O branching channel for the CH₃O₂ + HO₂ reaction has been performed using the turbulent flow technique with high‐pressure chemical ionization mass spectrometry for the detection of reactants and products. The temperature dependence of the CH₂O‐producing channel rate constant was investigated between 298 and 218 K at a pressure of 100 Torr, and the data were fitted to the following Arrhenius expression: 1.6 × 10^?15 × exp[(1730 ± 130)/T] cm³ molecule^?1 s^?1. Using the Arrhenius expression for the overall rate of the CH₃O₂ + HO₂ reaction and this result, the 298 K branching ratio for the CH₂O producing channel is measured to be 0.11, and the branching ratio is calculated to increase to a value of 0.31 at 218 K, the lowest temperature accessed in this study. The results are compared to the analogous CH₃O₂ + CH₃O₂ reaction and the potential atmospheric ramifications of significant CH₂O production from the CH₃O₂ + HO₂ reaction are discussed. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 363–376, 2001     

The oxidation of HI at low temperatures and the heat of formation of HO2

The kinetics of the oxidation of hydrogen iodide (HI + O₂) at low temperature (414–499 K) in the gas phase by the method of iodination kinetics is complicated by a heterogeneous reaction between hydrogen iodide and oxygen. Present work leads to an upper limit for the bimolecular rate constant k₁ for the first and rate-determining step (1) These data are combined with an estimated A factor A₁ = 10^9.3±0.2 L/mol·s (assuming a tight linear I···H···O— transition state), to calculate the lower limit of the activation energy for the forward reaction E₁. This leads to a minimum value for the heat of formation of the HO₂ radical, ΔH_f298^°(HO₂) < 3.0 kcal/mol.     

Some observations on the kinetics and thermochemistry of the reactions of HO2 radicals with aldehydes and ketones

The rapid, gas phase equilibrium addition of HO₂ radicals to CH₂O to form the peroxy radical HOCH₂OO^? is in agreement with the known thermochemistry of these species. The recent study of the similar addition of HO₂^? to ketones shows no significant reaction, which is again in agreement with known thermochemistry. All these reactions are notable for significant dipole attraction between the reactants ranging from 3 to 7 kcal/mol. The thermochemistry shows that the hydroperoxyl alkoxy species, the primary possible adduct, is not favored by the free energy change for direct addition. This and the observed kinetics favor a concerted addition, H‐atom transfer, as the transition state for the reactions. Kinetic estimates for forward and reverse reactions are in good agreement with observations. A thermochemical examination of the step‐wise addition of HO₂^? to the carbonyl shows that the reaction proceeds through a concerted, cyclic transition state involving simultaneous H‐transfer, 3 + 2 cyclo‐addition of HO₂^? to the carbonyl group. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 509–512, 2001     

The effect of methanol on the self reaction of HO2 radicals

The rate constant for the HO₂ self reaction has been determined as a function of methanol vapor concentration at 278 K and 299 K. A molecular modulation technique was used in which HO₂ radicals were photochemically produced in flowing gas mixtures comprised of Cl₂, CH₃OH, N₂ and O₂ with HO₂ monitored in the UV at 220 nm. A positive linear dependence of the second order rate constant on methanol concentration was found and this effect increased with decreasing temperature. The rate constant for the HO₂ self reaction can be described by in nitrogen at atmospheric pressure and in the methanol concentration range 1 · 10¹⁶ to 3 · 10¹⁷ molecules cm^?3.     

Temperature‐ and Pressure‐Dependent Kinetics of CH2OO + CH3COCH3 and CH2OO + CH3CHO: Direct Measurements and Theoretical Analysis

The rate coefficients of the gas‐phase reactions CH₂OO + CH₃COCH₃ and CH₂OO + CH₃CHO have been experimentally determined from 298–500 K and 4–50 Torr using pulsed laser photolysis with multiple‐pass UV absorption at 375 nm, and products were detected using photoionization mass spectrometry at 10.5 eV. The CH₂OO + CH₃CHO reaction's rate coefficient is ～4 times faster over the temperature 298–500 K range studied here. Both reactions have negative temperature dependence. The T dependence of both reactions was captured in simple Arrhenius expressions: The rate of the reactions of CH₂OO with carbonyl compounds at room temperature is two orders of magnitude higher than that reported previously for the reaction with alkenes, but the A factors are of the same order of magnitude. Theoretical analysis of the entrance channel reveals that the inner 1,3‐cycloaddition transition state is rate limiting at normal temperatures. Predicted rate‐coefficients (RCCSD(T)‐F12a/cc‐pVTZ‐F12//B3LYP/MG3S level of theory) in the low‐pressure limit accurately reproduce the experimentally observed temperature dependence. The calculations only qualitatively reproduce the A factors and the relative reactivity between CH₃CHO and CH₃COCH₃. The rate coefficients are weakly pressure dependent, within the uncertainties of the current measurements. The predicted major products are not detectable with our photoionization source, but heavier species yielding ions with masses m/z = 104 and 89 are observed as products from the reaction of CH₂OO with CH₃COCH₃. The yield of m/z = 89 exhibits positive pressure dependence that appears to have already reached a high‐pressure limit by 25 Torr.     

The UV absorption spectrum of the CCl3 radical and the kinetics of its mutual combination reaction from 253 to 623 K

The UV absorption spectrum and the kinetics of the self combination reaction of the CCl₃ radical were studied by flash photolysis in the temperature range 253–623 K. Experiments were performed at the atmospheric pressure, except for a few runs at the highest temperatures, which were performed between 30 and 760 torr. CCl₃ radicals were generated by flash photolysis of molecular chlorine in the presence of chloroform. The UV spectrum exhibits a strong unstructured band between 195 and 260 nm with a maximum at 211 ± 2 nm. The absorption cross section, measured relative to σ(HO₂), is σ(CCl₃) = (1.45 ± 0.35) × 10^?17 cm² molecule^?1 at the maximum. This value takes into account the uncertainty in σ(HO₂) which was taken equal to (4.9 ± 0.7) × 10^?18 cm² molecule^?1. The absolute rate constant for the CCl₃ mutual combination was determined by computer simulation of the transient decays. The rate constant, which exhibits a slight negative temperature coefficient, can be expressed as: The study of the pressure dependence showed that only a slight fall-off behavior could be observed at the highest temperature (623 K). This result was corroborated by RRKM calculations which showed that the rate constant is at the high pressure limit under most experimental conditions below 600 K.     

DFT and ab initio study on the reaction mechanism of CH2SH + O2

The mechanism for the CH₂SH + O₂ reaction was investigated by DFT and ab initio chemistry methods. The geometries of all possible stationary points were optimized at the B3LYP/6-311+G(d,p) level, and the single point energy was calculated at the CCSD(T)/cc-pVXZ(X = D and T), G3MP2 and BMC-CCSD levels. The results indicate that the oxidation of CH₂SH by O₂ to form HSCH₂OO is a barrierless process. The most favorable channel is the rearrangement of the initial adduct HSCH₂OO (IM1) to form another intermediate H₂C(S)OOH (IM3) via a five-center transition state, and then the C–O bond fission in IM3 leads to a complex CH₂S. . .HO₂ (MC1), which finally gives out to the major product CH₂S + HO₂. Due to high barriers, other products including cis- and trans-HC(O)SH + HO could be negligible. The direct abstraction channel was also determined to yield CH₂S + HO₂, with the barrier height of 22.3, 18.1 and 15.0 kcal/mol at G3MP2, CCSD(T)/cc-pVTZ and BMC-CCSD levels, respectively, it is not competitive with the addition channel, in which all stationary points are lower than reactant energetically. The other channels to produce cis- and trans-CHSH + HO₂ are also of no importance.     

Effect of ammonia,ammonia-water,and sulfuric acid on the HO2 + HO2 → H2O2 + 3O2 reaction in troposphere: Competition between stepwise and one-step mechanisms

A detailed theoretical study on the reaction mechanisms for the formations of H₂O₂ + ³O₂ from the self-reaction of HO₂ radicals under the effect of NH₃, H₃N···H₂O, and H₂SO₄ catalysts was performed using the CCSD(T)/CBS//M06-2X/aug-cc-pVTZ method. The rate constant was computed using canonical variational transition state theory (CVT) with small curvature tunneling (SCT). Our results indicate that NH₃-, H₃N···H₂O-, and H₂SO₄-catalyzed reactions could proceed through both one-step and stepwise routes. Calculated rate constants show that the catalyzed routes in the presence of the three catalysts all prefer stepwise pathways. Compared to the catalytic efficiency of H₂O, the efficiencies of NH₃, H₃N···H₂O, and H₂SO₄ are much lower due to their smaller relative concentrations. The present results have provided a definitive example of how basic and acidic catalysts influence the atmospheric reaction of HO₂ + HO₂ → H₂O₂ + ³O₂. These results further encourage one to consider the effects of basic and acidic catalysts on the related atmospheric reactions. Thus, the present investigation should have broad implications in the gas-phase reactions of the atmosphere.     

Angular momentum conservation in the O + OH ↔ O2 + H reaction

We have studied the O + OH ↔ O₂ + H reaction on Varandas's DMBE IV potential using a variety of statistical methods, all involving the RRKM assumption for the HO₂* complex. Comparing our results using microcanonical variational transition‐state theory (μVT) with those using microcanonical/fixed‐J variational transition‐state theory (μVT‐J), we find that the effect of angular momentum conservation on the rate coefficient is imperceptible up to a temperature of about 700 K. Above 700 K angular momentum conservation increasingly reduces the rate coefficient, but only by approximately 21% even at 5000 K. Comparing our μVT‐J calculations with the quasi‐classical trajectory (QCT) results of Miller and Garrett [ 1 ], we confirm their conclusion that non‐RRKM dynamics of the HO₂* complex reduces the rate coefficient by about a factor of 2 independent of temperature. Our calculations of k^(c), the rate coefficient for HO₂* formation from O + OH, are in excellent agreement with the QCT results of Miller and Garrett. Although the differences are not large, we find k_CVT^(c) > k_μVT^(c) > k_μVT‐J^(c) > k_QCT^(c), where CVT stands for canonical variational transition‐state theory. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 753–756, 1999     

Polarization dependence of near-resonant two-photon absorption of Na2 vapor

We report theoretically and experimentally on the polarization dependence of two-photon absorption of diatomic molecules for the case where only one intermediate rotationvibration level (off-resonance Δ) contributes to the transition strength. In this case, in the classical limit (J → ∞), the polarization between is (i) independent of the electronic symmetries, and (ii) it can be different for the regime Δ ? Δv_D and Δ ≈ Δν_D, due to velocity-tuning. The experiments have been performed on the 16601.88 cm^?1 two-photon transition of Na₂; they show that this transition belongs to the Q-branch.     

Variational transition‐state theory study of the rate constant of the DMS·OH scavenging reaction by O2

The chemical tropospheric dimethyl sulfide (DMS, CH₃SCH₃) degradation involves several steps highly dependent on the environmental conditions. So, intensive efforts have been devoted during the last years to enhance the understanding of the DMS oxidation mechanism under different conditions. The reaction of DMS with OH is considered to be the most relevant process that initiates the whole oxidation process. The experimental observations have been explained by a two‐channel mechanism consisting of a H‐abstraction process leading to CH₃S(O)CH₃ and HO₂ and an addition reaction leading to the DMS · OH adduct. In the presence of O₂, the DMS · OH adduct is competitively scavenged increasing the contribution of the addition channel to the overall DMS oxidation. Recent experimental measurements have determined from a global fit that the rate constant of this scavenging process is independent of pressure and temperature but this rate constant cannot be directly measured. In this article, a variational transition‐state theory calculation of the low‐ and high‐pressure rate constants for the reaction between DMS · OH and O₂ has been carried out as a function of temperature. Our proposal is that the slight temperature dependence of the scavenging rate constant can only be explained if the H‐abstraction bottleneck is preceded by a dynamical bottleneck corresponding to the association process between the DMS · OH adduct and the O₂ molecule. The agreement between the low‐pressure and high‐pressure rate constants confirms the experimental observations. © 2011 Wiley Periodicals, Inc. J Comput Chem, 2011     

An investigation of nonequilibrium kinetic isotope effects in chemically activated vinyl radicals

New experimental data have been obtained for H + C₂H₂, D + C₂H₂, H + C₂D₂, and D + C₂D₂ at room temperature. Two previously described apparatus were used in order to measure the pressure dependence of the reactions. The absolute rate constants are compared to results from other laboratories. The present results and those of Payne and Stief are used to obtain the high-pressure limiting rate constant at room temperature. When the activation energy from the work of Payne and Stief is considered, it is shown that the A factor for H + C₂H₂ is too low by a factor of ～20. If a transmission coefficient is introduced which is constant for all isotopic variations, the pressure dependence can be explained in terms of the randomly energized radicals. RRKM theory is then invoked to explain the observed statistical nonequilibrium kinetic isotope effects.