Jean-Claude Rayez

Gas phase oxidation of benzene: Kinetics, thermochemistry and mechanism of initial steps

A new investigation of the primary steps of the benzene oxidation, involving complementary experimental and theoretical approaches, is presented. The reactions of the OH-adduct (hydroxy-cyclohexadienyl radical c-C6H6-OH) were investigated using laser flash photolysis and producing OH radicals by H2O2 photolysis at 248 nm. It is confirmed that the adduct is in equilibrium with the corresponding peroxy radical RO2, near atmospheric conditions, the measured equilibrium constant being: Kc,2b = (2.62 ± 0.24) × 10−19 cm3 molecule−1 at 295 K, with the temperature dependent expression: ln(Kc,2b/cm3 molecule−1) = −63.29 + 6049/T, obtained by using the calculated entropy of reaction. The rate constant of the association reaction yielding RO2 is: k2b = (1.31 ± 0.12) × 10−15 cm3 molecule−1 s−1. Calculated data are in agreement with those values. In addition, data analysis shows that the reaction c-C6H6-OH + O2 involves an irreversible loss of radical species, yielding phenol and other oxidation products, with the global rate constant: kloss = (2.52 ± 0.40) × 10−16 cm3 molecule−1 s−1. Quoted errors are statistical (2σ), the possible total errors on the above values being estimated at around 40%. By comparison with the kloss value, the rate constant for phenol formation, calculated using a combination of DFT and ab initio CCSD(T) methods, corresponds to a phenol yield of about 55%, in reasonable agreement with experimental observations. Thermochemical and kinetic parameters have been calculated for the formation and for the reactions of the two RO2 stereoisomers, cis and trans. They show that the observed equilibrium must involve the trans isomer, which is more stable and is formed more rapidly than the cis isomer. Calculations show that the only possible reactions of peroxy radicals, under atmospheric conditions, is cyclisation yielding a bicyclic radical. However, cyclisation of the RO2(trans) is calculated to be too slow, compared to the rate of the irreversible radical loss, whereas it is very fast in the case of the cis isomer and can lead readily to oxidation products. On the basis of those results, a reaction mechanism is proposed for the first steps of benzene oxidation, consistent with all experimental and theoretical data, and which accounts for the principal oxidation products observed.

Water Vapor Effect on the HNO3 Yield in the HO2 + NO Reaction: Experimental and Theoretical Evidence

The influence of water vapor on the production of nitric acid in the gas-phase HO(2) + NO reaction was determined at 298 K and 200 Torr using a high-pressure turbulent flow reactor coupled with a chemical ionization mass spectrometer. The yield of HNO(3) was found to increase linearly with the increase of water concentration reaching an enhancement factor of about 8 at [H(2)O] = 4 x 10(17) molecules cm(-3) ( approximately 50% relative humidity). A rate constant value k(1bw) = 6 x 10(-13) cm(3) molecule(-1) s(-1) was derived for the reaction involving the HO(2)xH(2)O complex: HO(2)xH(2)O + NO --> HNO(3) (1bw), assuming that the water enhancement is due to this reaction. k(1bw) is approximately 40 times higher than the rate constant of the reaction HO(2) + NO --> HNO(3) (1b), at the same temperature and pressure. The experimental findings are corroborated by density functional theory (DFT) calculations performed on the H(2)O/HO(2)/NO system. The significance of this result for atmospheric chemistry and chemical amplifier instruments is briefly discussed. An appendix containing a detailed consideration of the possible contribution from the surface reactions in our previous studies of the title reaction and in the present one is included.

Isomerisation reactions of alkoxy radicals: theoretical study and structure–activity relationships

Thermochemical and kinetic parameters for 1,5-H isomerisation reactions of alkoxy radicals up to C8 have been determined theoretically using density functional theory. Pressure dependence (through RRKM statistical calculations) as well as tunneling corrections have been taken into account. The results of calculations are validated by available experimental relative rate constants. These results show that the set of alkoxy radicals studied can be divided into three categories according to the H-abstraction site involved in the isomerisation reaction (primary, secondary and tertiary). Values for kinetic parameters: pre-exponential factors, activation energies and rate constants are proposed for each category. In particular, the following rate constant values are predicted: kisom = 6.2 × 105 s−1, 9.3 × 106 s−1 and 4.5 × 108 s−1 for 1,5-H transfer from a primary group (–CH3), secondary group (–CH2–) and tertiary group (>CH–), respectively, at 298 K and 1 atm pressure. An uncertainty factor of about 5 is estimated for calculated rate constants. These results corroborate Atkinsons recommendations except for the third group for which our value is two orders of magnitude larger. Another result of this study is that the pressure dependence of the rate constant for the isomerisation reaction is weak except for abstraction of a tertiary H-atom where kisom (298 K, 1 atm) is 40% of the infinite pressure rate constant. It can be also stressed that, where the isomerisation is possible, it will always be the dominant pathway with respect to the reaction with O2, but it may be in competition with the decomposition reaction. We show that this is also the case in upper tropospheric conditions (0.2 atm and 220 K).

Theoretical study on the atmospheric fate of carbonyl radicals: kinetics of decomposition reactions

The decomposition kinetics of a large set of representative R–CO carbonyl radicals has been studied theoretically. These radicals can either decompose into R + CO or, in the presence of oxygen, add to O2 to give acylperoxy radicals RC(O)O2. In this work, it is shown, by comparison to available experimental data, that reliable quantitative kinetic parameters for decomposition reactions can be obtained using DFT (B3LYP and BH&HLYP functionals) and ab initio G2(MP2) methods. Moreover, it has been demonstrated, by performing RRKM calculations, that the dissociation of carbonyl radicals under atmospheric conditions is not only governed by the height of the barriers, but that pressure effects can also play an important role and must be taken into account. Structure–activity relationships for the decomposition of R–CO radicals are presented according to the nature of the R group. The R–CO radicals, which are predicted to decompose under atmospheric conditions, contain chlorine or oxygen in the R group.

An experimental and theoretical investigation of the gas-phase benzene–OH radical adduct + O2 reaction

The reaction of the hydroxycyclohexadienyl radical (HO–C6H6) (the adduct from the benzene + OH reaction) with O2 has been investigated using laser flash photolysis with UV-absorption spectroscopic detection, and DFT and ab initio quantum mechanical calculations. An absolute absorption spectrum was measured for the benzene–OH adduct, and its reaction with O2, giving a peroxy radical species, was seen to be equilibrated around room-temperature. An equilibrium constant of 1.15 ± 0.6 × 10−19 cm3 molecule−1 was determined at 295 K from an analysis of transient absorption signals using a detailed reaction mechanism. Equilibrium constants were obtained in this way at six different temperatures between 265 and 345 K. The temperature-dependence of these data indicates that the ΔH0298 and ΔS0298 for the title reaction are −10.5 ± 1.3 kcal mol−1 and −33.9 ± 1.4 cal K−1 mol−1 respectively (second-law analysis of the data, 2σ errors). A third-law analysis of the data (using a value for ΔS0298 of −38.3 cal K−1 mol−1, derived from DFT and ab initio calculations) yields a value for ΔH0298 of −11.7 ± 0.2 kcal mol−1, which compares with an ab initio calculated value of −12.2 kcal mol−1. Absorption signals at 260–275 nm, in the presence of high concentrations of O2, were observed that are consistent with the presence of the benzene–OH peroxy radical, and with stable products of its chemistry. Equilibrium constants obtained from these data agree well with our other determinations. The effective lifetime of the equilibrium system—adduct + O2 ⇌ adduct − O2—is dictated either by an additional, irreversible reaction of the benzene–OH adduct with O2 or by a unimolecular transformation of the peroxy species. Assuming the former case, a bimolecular rate constant of around 5.5 ± 3.0 × 10−16 cm3 molecule−1 s−1 was estimated from a kinetic simulation of our decay signals. This rate constant does not appear to vary significantly between 265 and 320 K, but it must be emphasised that it was estimated with a fairly high uncertainty.

Reaction of methylidyne radical with CH4 and H2S: overall rate constant and absolute atomic hydrogen production

Abstract The CH+CH4 and H2S reactions were studied, at room temperature, in a low-pressure fast-flow reactor. CH ( X 2 Π, v=0) radicals were obtained from the reaction of CHBr3 with potassium atoms. The overall rate constants were found at 330 K to be (0.76±0.20)×10−10 and (2.8±0.8)×10 −10 cm 3 molecule −1 s −1 , respectively. The absolute atomic hydrogen productions were determined by resonance fluorescence in the vacuum ultraviolet: H production from the CH+CH4 reaction is 100% and from the CH+H2S reaction is 99+1−4%, the H production from the CH+H2 reaction being the reference. Ab initio studies of the different stationary points relevant to the CH+CH4 reaction have been performed at the CCSD(T)/cc-pVTZ level and comparison is made with experimental results. The experimental results for the CH+H2S reaction is compared with those of a recent theoretical study [Chem. Phys. 242 (1999) 1].

Clustering of water molecules on model soot particles: an ab initio study

Clustering of water molecules on model soot particles is studied by means of quantum calculations based on the ONIOM approach. The soot particles are modelled by anchoring OH or COOH groups on the face side or on the edges of a graphite crystallite of nanometer size. The quantum calculations aim at characterizing the adsorption properties (structure and adsorption energy) of small water aggregates containing up to 5 water molecules, in order to better understand at a molecular level the role of these OH and COOH groups on the behavior with respect to water adsorption of graphite surface modelling soot emitted by aircraft.

A reinvestigation of the kinetics and the mechanism of the CH3C(O)O2+ HO2 reaction using both experimental and theoretical approaches

The kinetics and the mechanism of the reaction CH(3)C(O)O(2)+ HO(2) were reinvestigated at room temperature using two complementary approaches: one experimental, using flash photolysis/UV absorption technique and one theoretical, with quantum chemistry calculations performed using the density functional theory (DFT) method with the three-parameter hybrid functional B3LYP associated with the 6-31G(d,p) basis set. According to a recent paper reported by Hasson et al., [J. Phys. Chem., 2004, 108, 5979-5989] this reaction may proceed by three different channels: CH(3)C(O)O(2)+ HO(2)--> CH(3)C(O)OOH + O(2) (1a); CH(3)C(O)O(2)+ HO(2)--> CH(3)C(O)OH + O(3) (1b); CH(3)C(O)O(2)+ HO(2)--> CH(3)C(O)O + OH + O(2) (1c). In experiments, CH(3)C(O)O(2) and HO(2) radicals were generated using Cl-initiated oxidation of acetaldehyde and methanol, respectively, in the presence of oxygen. The addition of amounts of benzene in the system, forming hydroxycyclohexadienyl radicals in the presence of OH, allowed us to answer that channel (1c) is <10%. The rate constant k(1) of reaction (1) has been finally measured at (1.50 +/- 0.08) x 10(-11) cm(3) molecule(-1) s(-1) at 298 K, after having considered the combination of all the possible values for the branching ratios k(1a)/k(1,)k(1b)/k(1,)k(1c)/k(1) and has been compared to previous measurements. The branching ratio k(1b)/k(1), determined by measuring ozone in situ, was found to be equal to (20 +/- 1)%, a value consistent with the previous values reported in the literature. DFT calculations show that channel (1c) is also of minor importance: it was deduced unambiguously that the formation of CH(3)C(O)OOH + O(2) (X (3)Sigma(-)(g)) is the dominant product channel, followed by the second channel (1b) leading to CH(3)C(O)OH and singlet O(3) and, much less importantly, channel (1c) which corresponds to OH formation. These conclusions give a reliable explanation of the experimental observations of this work. In conclusion, the present study demonstrates that the CH(3)C(O)O(2)+ HO(2) is still predominantly a radical chain termination reaction in the tropospheric ozone chain formation processes.

Mean potential phase space theory of chemical reactions.

A nonconventional application of phase space theory to the insertion reactions A+H(2), with A=C((1)D) and S((1)D), is presented. Instead of approximating the potential energies of interaction between separated fragments by their isotropic long-range contributions, as in the original theory, the latter are replaced by the accurate potential energies averaged with respect to Jacobi angles. The integral and differential cross sections obtained from this mean potential phase space theory (MPPST) turn out to be in very satisfying agreement with the benchmark predictions of the time-independent and time-dependent statistical quantum methods. The formal and numerical simplicity of MPPST with respect to any approach combining statistical assumptions and dynamical calculations makes it a promising tool for studying indirect polyatomic reactions.

Rotational excitation and de-excitation of HF molecules by He atoms

We report Close Coupling calculations of the rotational transitions induced in collisions between HF molecules and He atoms. We consider transitions for levels up to J = 9 and for temperatures up to 300 K. We use a new global 3D potential energy surface which we proposed recently and is the best available at the moment. This surface exhibits two minima associated both with linear geometries (V = -43.70 cm -1 for He-HF and V = -25.88 cm -1 for He-FH). Close Coupling calculations are performed in the collision energy 10 -6 to 2000 cm -1 interval. Our results validate the estimate, by Neufeld et al., of the abundance of HF in the interstellar medium which was detected recently.