F. Caralp

Investigation into the pressure dependence between 1 and 10 Torr of the reactions of NO2 with CH3 and CH3O

The kinetics and pressure dependence of the reactions of NO2 with CH3 and CH3O have been investigated in the gas phase at 298 K, at pressures from 1 to 10 Torr. A low-pressure discharge-flow laser-induced fluorescence (LIF) technique was used. In a consecutive process, CH3 reacted with NO2 to form CH3O, CH3+ NO2→ CH3O + NO (1), which further reacted with NO2 to form products, CH3O + NO2→ products (2). Reaction (1) displayed no discernible pressure dependence over the pressure range 1–7 Torr, and k1 was calculated to be (2.3 ± 0.3)× 10–11 cm3 molecule–1 s–1. Reaction (2) displayed a strong pressure dependence and an RRKM analysis yielded the following limiting low- and high-pressure rate constants in He, k0= 5.9 × 10–29 cm6 molecule–2 s–1 and k∞= 2.1 × 10–11 cm3 molecule–1 s–1. It is unrealistic to quote errors for this type of analysis. Parametrisation in the standard Troe form with Fc= 0.6 yielded k0=(5.3 ± 0.2)× 10–29 cm6 molecule–2 s–1 and k∞=(1.4 ± 0.1)× 10–11 cm3 molecule–1 s–1. Atmospheric implications and possible reaction mechanisms are discussed.

Gas‐Phase Kinetics of Hydroxyl Radical Reactions with Alkenes: Experiment and Theory

Reactions of the hydroxyl radical with propene and 1-butene are studied experimentally in the gas phase in a continuous supersonic flow reactor over the range 50≤T/K≤224. OH radicals are produced by pulsed laser photolysis of H(2)O(2) at 266 nm in the supersonic flow and followed by laser-induced fluorescence in the (1, 0) A(2)Σ(+)←X(2)Π(3/2) band at about 282 nm. These reactions are found to exhibit negative temperature dependences over the entire temperature range investigated, varying between (3.1-19.2) and (4.2-28.6)×10(-11) cm(3) molecule(-1) s(-1) for the reactions of OH with propene and 1-butene, respectively. Quantum chemical calculations of the potential energy surfaces are used as the basis for energy- and rotationally resolved Rice-Ramsperger-Kassel-Marcus calculations to determine the rate constants over a range of temperatures and pressures. The negative temperature dependences of the rate constants are explained by competition between complex redissociation and passage to the adducts by using a model with two transition states. The results are compared and contrasted with earlier studies and discussed in terms of their potential relevance to the atmosphere of Saturn.

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.

Tunneling in the reaction of acetone with OH

Based on recent detailed quantum mechanical computations of the mechanism of the title reaction and, this paper presents kinetics analysis of the overall rate constant and its temperature dependence, for which ample experimental data are available for comparison. The analysis confirms that the principal channel is the formation of acetonyl radical + H(2)O, while the channel leading to acetic acid is of negligible importance. It is shown that the unusual temperature dependence of the overall rate constant, as observed experimentally, is well accounted for by standard RRKM treatment that includes tunneling. This treatment is applied at the microcanonical level, with chemically activated distribution of entrance species, i.e. using a stationary rather than a thermal distribution that incorporates collisional energy transfer and competition between the redissociation and exit channel. A similar procedure is applied to the isotopic reaction acetone-d6 + OH with equally satisfying results, so that the experimental temperature dependence of the KIE (kinetic isotope effect) is perfectly reproduced. This very good agreement between calculation and experiment is obtained without any fitting to experimental values and without any adjustment of the parameters of calculation.

Flash photolysis study of the CH3O2+ CH3O2 and CH3O2+ HO2 reactions between 600 and 719 K: unimolecular decomposition of methylhydroperoxide

The reactions: CH3O2+ CH3O2→ 2CH3O + O2(1a), → CH3OH + HCHO + O2(1b) and CH3O2+ HO2→ CH3OOH + O2(2) have been studied between 600 and 719 K and at atmospheric pressure, using the flash photolysis/UV absorption method. The peroxy radicals were generated via the photolysis of molecular oxygen around 200 nm in the presence of CH4(for CH3O2) and/or CH3OH (for HO2). Results for k1 and k2 are in good agreement with earlier lower-temperature work in this laboratory. Reanalysis of all results from this laboratory to date, using recently reported temperature-dependent absorption cross-sections for CH3O2 and HO2 gives, for the temperature range 248–700 K: k1/cm3 molecule–1 s–1=(1.0 ± 0.1)× 10–13 exp[(416 ± 32)K/T], k2/cm3 molecule–1 s–1=(2.9 ± 0.3)× 10–13 exp[(862 ± 44)K/T] Above 600 K, the unimolecular decomposition of methylhydroperoxide becomes important: CH3OOH + M → CH3O + OH + M (3) Under most conditions, CH3O and OH are converted rapidly to HO2 and CH3O2 respectively, effectively reversing reaction (2). The occurrence of reaction (3) alters the form and increases the timescale of the radical decay, as the only remaining termination reaction for hydroperoxy radicals is their self-reaction. An Arrhenius fit gives: k3/s–1= 10(14.8±0.7) exp[–(177 ± 9)kJ mol–1/RT] Errors are 1σ.

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.

Atmospheric chemistry of benzaldehyde: UV absorption spectrum and reaction kinetics and mechanisms of the C6H5C(O)O2 radical

Flash photolysis–UV absorption and long pathlength FTIR–smog chamber studies of several reactions involving C6H5C(O) and C6H5C(O)O2 radicals have been performed. It was determined that reaction of Cl atoms with C6H5CHO proceeds via abstraction of the aldehydic hydrogen to give benzoyl radicals. The sole atmospheric fate of benzoyl radicals is addition of O2 to give peroxybenzoyl radicals. Reaction of C6H5C(O) radicals with molecular chlorine proceeds with a rate constant of (5.9±0.4)×10-11 cm3 molecule-1 s-1 at 296 K and 1–700 Torr total pressure. The UV spectrum of C6H5C(O)O2 radicals (245–300 nm) and the self reaction were investigated simultaneously, yielding σmax=(2.0±0.1)×10-17 cm2 molecule-1 at 245 nm and k16=(3.1±1.4)×10-13 exp[(1110±160) K/T] cm3 molecule-1 s-1, measured from 298 to 460 K. At 338 K, C6H5C(O)O2 radicals react with NO with a rate constant of (1.6±0.4)×10-11 cm3 molecule-1 s-1. At 296 K, C6H5C(O)O2 radicals react with NO2 with a rate constant of (1.1±0.3)×10-11 cm3 molecule-1 s-1 to form C6H5C(O)O2NO2, which undergoes thermal decomposition at a rate of k-4=(2.1-1.5+5.0)×1016 exp[-(13600±400)K/T] s-1 in one atmosphere of air. At 296 K in 100–700 Torr of air k[C6H5C(O)O2+NO]/k[C6H5C(O)2+NO2]=1.44±0.15. Relative rate methods were used to measure k[Cl+C6H5C(O)Cl]=(1.1±0.2)×10-15 cm3 molecule-1 s-1 at 296 K. Uncertainty limits are all two standard deviations. Results are discussed with respect to the literature data and the atmospheric chemistry of benzaldehyde.

Photo-oxidation of halomethanes at low temperature: the decomposition rate of CCl3O and CFCl2O radicals

Abstract The chlorine-photosensitized oxidation chains of CHCl 3 and CHFCl 2 were studied at 233 K and 253 K respectively to evaluate the decomposition rate of CCl 3 O and CFCl 2 O radicals: Various amounts of NO were introduced into the system to quench the chain reactions by introducing competition with the above reactions. In spite of the low temperature, these reactions were found to be fast and only lower limits of the rate constants could be determined: 1 × 10 5 s −1 at 233 K and 3 × 10 4 s −1 at 253 K for CCl 3 O and CFCl 2 O respectively at low pressure (about 7 Torr). This result shows that reactions other than these decompositions are highly unlikely for these species, either in the stratosphere or in any other reacting system.

Microcanonical variational theory of radical recombination by inversion of interpolated partition function. Part 2.—CX3+ O2(X = H, F, Cl)

The title theory (MVIPF) has been applied to the recombinations of O2 with the radicals CH3, CF3, CF2Cl, CFCl2 and CCl3 and compared, with generally good results, to experimental data which are available for all radicals but one (CF2Cl). In particular, MVIPF, which is based on minimum information and uses an adjustable parameter (c) in a Gaussian-type switching function, correctly reproduces the fact that the recombination CH3+ O2 has a positive temperature coefficient, while all the other radical recombinations have a negative temperature coefficient. Consolidated results by MVIPF for the nine recombination reactions for which experimental data are available (the title reactions, plus CH3+ H, CH3+ CH3, CH2Cl + CH2Cl, CHCl2+ CHCl2, CCl3+ CCl3), yield a linear relation for a function of the logarithm of c, which in principle allows one to predict a recombination rate constant in a series of similar reactions within a factor of two.