Marie-Thérèse 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.

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.

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.

Theoretical study on the comparative fate of the 1-butoxy and β-hydroxy-1-butoxy radicals

Theoretical high level ab initio BAC-MP4 and DFT calculations followed by a kinetic RRKM analysis have been performed in this work for the study of unimolecular reactions of the 1-butoxy and β-hydroxy-1-butoxy radicals. We have shown that the substitution of H by OH on the carbon in the β position of the 1-butoxy radical (leading to the β-hydroxy-1-butoxy radical) results in an important lowering of the decomposition barrier and a slight increase of the isomerisation barrier. Coupled to the rate constant calculations, this study suggests that, contrary to the fate of the 1-butoxy radical, the thermal decomposition is the major pathway for the β-hydroxy-1-butoxy radical. We have also shown that, under atmospheric conditions (760 Torr and 298 K), both isomerisation and decomposition processes are still in the fall-off range for the hydroxy radical. These behaviors have been interpreted in terms of electronic structures and intramolecular hydrogen bonding. This is the first theoretical study of the β-hydroxy-1-butoxy radical unimolecular reactions. As there are no experimental measurements on the β-hydroxy-1-butoxy radical rate constants, this theoretical study is the first to predict kinetic parameters for the decomposition and isomerisation reactions of this compound.

Kinetics and mechanism of the gas-phase reaction of the cyclohexadienyl radical c-C6H7 with O2

The kinetics and mechanism of the gas-phase reaction of the cyclohexadienyl radical c-C6H7 with O2 have been investigated using both experimental and theoretical approaches. The rate constant has been measured using conventional flash photolysis in the temperature range 302–456 K, 1 atm pressure. c-C6H7 radicals were produced by reacting Cl atoms with 1,4-cyclohexadiene. The rate expression is k1=(1.4±0.26)×10−13 exp[−(300±74) K/T] cm3 molecule−1 s−1 (2σ error bars). The reaction can proceed either by association, yielding a peroxy radical RO2 or by H-abstraction, yielding benzene+HO2, the two reaction channels involving two distinct transition states. In contrast to what is observed for the c-C6H6OH radical, no equilibrium with the peroxy radical could be characterised. The theoretical approach, involving both DFT and ab initio methods, was used to determine if the measured rate constant should be assigned to the association or to the H-abstraction channel. Comparison of experimental and theoretical results shows that H-abstraction must be the only significant reaction channel.

Kinetics of the combination reactions of chlorofluoromethylperoxy radicals with NO2 in the temperature range 233–373 K

The rate parameters for the combination reactions of CCl3O2, CCl2FO2 and CF3O2 radicals with NO2 were measured in the pressure range 1–10 Torr and from 233 to 373 K. Experiments were performed by pulsed laser photolysis and time-resolved mass spectrometry. Al reactions are in the fall-off region and are significantly faster than the equivalent reaction of CH3O2. They exhibit strong negative temperature coefficients and the rate constants increase in the series from CF3O2 to CCl3O2. The experimental equilibrium constant was obtained for the reaction of CCl2FO2 at 273 K, by using previously determined rate constants for the reverse reaction. This determination allowed us to obtain ΔH° of the reaction by calculating the structural and spectroscopic parameters by the semi-empirical MNDO method. Assuming that the value of ΔH° is the same for all reactions of the series it was possible to calculate the temperature dependence of all equilibrium constants and the bond dissociation energies D°298(CX3O2—NO2)= 105 ± 5 kJ mol–1(X = For Cl). ARRKM model was set up for these reactions and calibrated for the reaction of CCl2FO2. The model is shown to reproduce quite adequately the fall-off curves and with the substitution of chlorine for fluorine in the series of radicals investigated. This model was used to extrapolate the low-pressure data to high pressure, where no experimental data were available, and to calculate the kinetic parameters for the CClF2O2 reaction, which could not be investigated experimentally in the present study. Kinetic parameters are reported for the association reaction of the entire series of chlorofluoromethylperoxy radicals with NO2.

Gas-phase detection of the HBCC (X1Σ) molecule: a combined crossed beam and computational study of the B(2P)+C2H2(1Σg+) reaction

A novel supersonic beam of ground‐state boron atoms [B(2P)] was employed to investigate the reaction of B(2P) with acetylene [C2H2(1Σg+)] at an average collision energy of 16.3±0.4 kJ mol−1 at the most fundamental microscopic level. The crossed molecular beam technique was used to record time of flight spectra at mass to charge ratios of 36 (11BC2H+), 35 (10BC2H+/11BC2+), and 34 (10BC2+) at different laboratory angles. Forward‐convolution fitting of the laboratory data showed that only a product with the gross formula BC2H was formed via a boron versus hydrogen exchange. By combining experimental results with electronic structure calculations, the conclusion was that the reaction proceeded via the initial addition of B(2P) to the two carbon atoms of acetylene, leading to the formation of a first intermediate, the borirene radical (c‐BC2H2). This intermediate underwent various isomerization processes on the BC2H2 potential energy surface before decomposing into the linear HBCC(X1Σ) isomer via a hydrogen atom elimination.

Water Adsorption on Oxidized Single Atomic Vacancies Present at the Surface of Small Carbonaceous Nanoparticles Modeling Soot

Quantum calculations are used to study the interaction of water molecules with carbonaceous clusters containing one single carbon atom vacancy. This is a simple but realistic way to model the active surfaces found in soot emitted by aircrafts. Prior to water adsorption, the atomic vacancy is oxidised by an approaching oxygen molecule, which is also likely to occur behind planes. The results of the calculations show that this oxidation process results in the formation of one ketone-like site and one epoxide-like site around the atomic vacancy. These sites may act as nucleation centers for water molecules, which are, however, physisorbed on the oxidized surface, leading to very weak charge transfer with the surface. Although less attractive for water than, for instance, a carboxyl-like site, the ketone-like site can also participate in the hydrophilic behavior of soot primary particles. In contrast, the epoxide-like site formed around the vacancy shows a very low affinity for water molecules.