Silvia I. Lane

Kinetic Study of OH Radical Reactions with CF3CClCCl2, CF3CClCClCF3 and CF3CF=CFCF3

The relative rate technique has been used to determine the rate constants of the reactions of OH radicals with CF(3)CCl=CCl(2) (k(1)), CF(3)CCl=CClCF(3) (k(2)) and CF(3)CF=CFCF(3) (k(3)). Experiments were carried out at (298±2) K and atmospheric pressure using ultrapure nitrogen as gas bath. The decay rates of the organic species were measured relative to those of ethane, methanol, acetone, chloroethane and 2-butanone. The following rate constants were derived in units of cm(3) molecule(-1) s(-1): k(1)= (10±1)×10(-13), k(2)=(2.1±0.2)×10(-13) and k(3)=(3.7±0.2)×10(-13). This is the first experimental determination of k(1) and k(2). The rate constants obtained are compared with previous literature data to establish reactivity trends and are used to estimate the atmospheric lifetimes of the studied perhaloalkenes. From the calculated lifetimes, using an average global concentration of hydroxyl radicals, the atmospheric loss of these compounds by the OH-initiated oxidation was determined. Also, estimations have been made of the ozone depletion potential (ODP), the radiative forcing efficiency (RE), the halocarbon global warming potential (HGWP) and the global warming potential (GWP) of the perhaloalkenes. The approximate nature of these values is stressed considering that these are short-lived compounds for which these atmospheric parameters may vary according to latitude and season.

Theoretical study of the relative reactivity of chloroethenes with atmospheric oxidants (OH, NO3, O(3P), Cl(2P) and Br(2P))

The reactivity of a series of chloroethenes with different electrophiles of tropospheric and stratospheric interest is analyzed by frontier molecular orbital theory and a correlation with calculated orbital energies is investigated. The reactions of CH2CHCl; CH2CCl2; (Z)-CHClCHCl; (E)-CHClCHCl; CHClCCl2 and CCl2CCl2 with O(3P), Cl(2P), Br(2P) atoms and with OH and NO3 radicals were studied using semiempirical methods (AM1 and PM3) and ab initio calculations at the HF and B3LYP levels of theory with the 6-31G** basis set, using the Gaussian 98 suite of programs. In contrast to the majority of reaction series of small radicals and molecules with alkenes and alkanes, the rate constants for the reactions with halogenated ethenes do not correlate with the ionization potential of the halogenated ethene. The energy and the carbon–carbon π-bonding form of the HOMO change on addition of chlorine atoms as substituents to the carbon–carbon σ-bonding framework of the alkenes. For the reactions studied the complete interaction HOMO–SOMO was considered, taking into account the contribution of the different atomic orbitals to the HOMO of the chloroethene through the atomic orbital coefficients, and a good correlation with the experimental values was obtained.

Reaction rate coefficients of OH radicals and Cl atoms with ethyl propanoate, n-propyl propanoate, methyl 2-methylpropanoate, and ethyl n-butanoate.

Kinetics of the reactions of OH radicals and Cl atoms with four saturated esters have been investigated. Rate coefficients for the gas-phase reactions of OH radicals with ethyl propanoate (k(1)), n-propyl propanoate (k(2)), methyl 2-methylpropanoate (k(3)), and ethyl n-butanoate (k(4)) were measured using a conventional relative rate method and the pulsed laser photolysis-laser induced fluorescence technique. At (296 +/- 2) K, the rate coefficients obtained by the two methods were in good agreement. Significant curvatures in the Arrhenius plots have been observed in the temperature range 243-372 K for k(1), k(3), and k(4). The rate coefficients for the reactions of the four esters with Cl atoms were determined using the relative rate method at (296 +/- 2) K and atmospheric pressure. The values obtained are presented, compared with the literature values when they exist, and discussed. Reactivity trends and atmospheric implications for these esters are also presented.

Ab initio studies of the gas-phase thermodynamic properties and bond dissociation energies for haloethanes and halomethyl radicals

Ab initio molecular orbital calculations have been performed on fluorine- and chlorine-substituted ethanes and radicals. The molecular equilibrium characteristics (energies, structural parameters and vibrational frequencies) of all the species were estimated at the MP2/6-311G** level of theory. A rigid harmonic oscillator model was used for the development of a data base for the ideal gas thermodynamic properties: Cp°, S°, − (G° − H0°)/T, ΔfH°, ΔfG° and log Kf, of the haloethanes and halomethyl radicals over a temperature range 0–1500 K. From the estimated enthalpies of formation at 298.15 K and 1 atm, bond dissociation energies for the homolytic cleavage of C–C bonds for the haloethanes were obtained. The results of the MP2/6-311G** calculations of the heats of formation at 298.15 K, ΔfH298.15K°, of the halogenated methyl radicals have been compared with those obtained using the CBS-RAD procedure and a modification of this called CBS-RAD(B3-LYP,B3-LYP). Finally, the influence of halogen substituents on reactivity has been interpreted in terms of polar and hyperconjugation effects.

Production of silicon containing particles by laser induced reaction of silane with methane, ethane and acetylene

Abstract Silicon containing ultrafine particles (Si and SiC) have been obtained by pulsed IR laser irradiation of gaseous SiH 4 /hydrocarbon (CH 4 , C 2 H 6 and C 2 H 2 ) mixtures. The chemical composition and structure of the powders formed were determined by scanning and transmission electron microscopy (SEM and TEM), X-ray diffraction (XRD) and I.R. and UV-v spectroscopy. The particles are small, uniform, loosely agglomerated spheres with a mean size of approximately 19 nm. To complete the characterisation of these ultrafine silicon-containing particles, the X-ray absorption spectra (XAS) were obtained using synchrotron radiation at the silicon absorption K-edge. We show that the use of pulsed laser radiation results in the generation of amorphous Si and crystalline SiC particles depending on the gas mixtue used, among other parameters such as composition of the mixture, laser fluence, pressure and number of laser pulses.

Water Catalysis of the Reaction between Methanol and OH at 294 K and the Atmospheric Implications

The rate coefficient for the reaction CH3 OH+OH was determined by means of a relative method in a simulation chamber under quasi-real atmospheric conditions (294 K, 1 atm of air) and variable humidity or water concentration. Under these conditions, a quadratic dependence of the rate coefficient for the reaction CH3 OH+OH on the water concentration was found. Thus the catalytic effect of water is not only important at low temperatures, but also at room temperature. The detailed mechanism responsible of the reaction acceleration is still unknown. However, this dependence should be included in the atmospheric global models since it is expected to be important in humid regions as in the tropics. Additionally, it could explain several differences regarding the global and local atmospheric concentration of methanol in tropical areas, for which many speculations about the sinks and sources of methanol have been reported.

Theoretical calculations of the kinetics of the OH reaction with 2-methyl-2-propen-1-ol and its alkene analogue

In this work, the first and rate determining steps of the mechanism of the OH addition to 2-methyl-2-propen-1-ol (MPO221) and methylpropene (M2) have been studied at the DFT level, employing the BH and HLYP functional and the cc-pVDZ and aug-cc-pVDZ basis sets. The thermochemical properties of equilibrium (enthalpy, entropy and Gibbs free energies) have been determined within the conventional statistical thermodynamics relations and the rate coefficients have been determined on the basis of the variational transition state theory. The adoption of the microcanonical variational transition state theory was proved to be crucial for the description of the kinetics of OH addition to these unsaturated compounds. The rate coefficients obtained for the OH reactions with MPO221 and M2 at 298.15 K deviate, respectively, 27% and 13% from the experimental rate coefficient available in the literature. A non-Arrhenius profile is observed for the rate coefficients. Moreover, the values of the rate coefficients for the MPO221 + OH reaction are greater than those for the M2 + OH reaction, suggesting that the substitution of the hydrogen atom in an alkene by the –OH functional group increases the reactivity with respect to the hydroxyl radical.

Comparative kinetics of the 3-buten-1-ol and 1-butene reactions with OH radicals: a density functional theory/RRKM investigation.

The compared kinetics of the reactions of unsaturated alcohols and alkenes with OH radicals is a topic of great interest from both the theoretical chemistry and the atmospheric chemistry points of view. The enhanced reactivity of an unsaturated alcohol, with respect to its alkene analogue, toward OH radicals has been previously demonstrated, at 298 K, by experimental and theoretical research. In this work, a new comparative investigation of such reactions is performed for 3-buten-1-ol and 1-butene. The model assumes that the overall kinetics is governed by the first OH addition steps of the mechanism. Calculations have been performed at the DFT level, employing the BHandHLYP functional and the cc-pVDZ and aug-cc-pVDZ basis sets, and the rate coefficients have been determined on the basis of the microcanonical variational transition state theory. The rate coefficients obtained for the OH reactions with 3-buten-1-ol (kOH(31BO)) and 1-butene (kOH(1B)) at 298.15 K are lower than the experimental rate coefficient available in the literature, showing deviations of 18% and 25%, respectively. Negative temperature dependence is verified for these rate coefficients. The kOH(31BO)/kOH(1B) ratios have also been investigated as a function of the temperature, suggesting that at room temperature the unsaturated alcohol reacts with the OH radicals faster than 1-butene, by a factor of 1.2, but at higher temperatures (400-500 K), the alkene should react faster, and that the stabilization of prebarrier complexes and saddle points due to hydrogen bonds is no longer an important factor to govern the reactivity of the unsaturated alcohol toward OH radicals, with respect to the alkene analogue.

A theoretical study of hydrogen and chlorine transfer reactions from fluorine- and chlorine-substituted methanes by F3C˙ radicals

The minimum energy path for the hydrogen and chlorine abstraction reactions from CX4−nHn (n = 1, 2 or 3; X = Cl or F) and CCl4−nFn (n = 0, 1 or 2) series by trifluoromethyl radicals (F3C˙) has been determined using ab initio molecular orbital calculations. All the structural parameters (geometry and vibrational frequencies) were computed at the B3LYP/6-31G(d) level of theory. The characteristics of the transition states for the hydrogen atom transfer reactions from the halomethane series, CX4−nHn (n = 1, 2 or 3; X = Cl or F), were also determined using the MP2/6-31G(d,p) level of theory. This database was then used to calculate the kinetic parameters by means of the transition-state theory. The results indicated that the density functional theory (DFT) approach can provide energies that are comparable to most of the experimentally derived values. However, to obtain a better reference and to test the reliability of the activation barriers, we have also carried out computations using the MP4(SDQ)(fc)/6-31+G(d,(f),d,p)//MP2/6-31G(d,p) approach and the CBS-RAD(B3LYP-B3LYP) procedure on the reactions involving the CX4−nHn (n = 1 or 2; X = Cl or F) halomethanes. In addition, the reactivity trends of these species were interpreted in terms of a balance between the relative strength of the bonds being broken and formed and the polarity variations at the transition state.

Kinetic study of the OH and Cl-initiated oxidation, lifetimes and atmospheric acceptability indices of three halogenated ethenes

The gas-phase kinetics for the reactions of OH radicals and Cl atoms with (E/Z)-CHClCHF, (E/Z)-CFClCFCl, and CCl2CF2 were investigated at room-temperature and atmospheric pressure. A conventional relative-rate technique was used to determine the rate coefficients k(OH + (E/Z)-CHClCHF) = (6.3 ± 1.2) × 10−12, k(OH + (E/Z)-CFClCFCl) = (1.6 ± 0.2) × 10−12, k(OH + CCl2CF2) = (5.0 ± 0.7) × 10−12, k(Cl + (E/Z)-CHClCHF) = (11 ± 2) × 10−11, k(Cl + (E/Z)-CFClCFCl) = (5.4 ± 1.3) × 10−11, and k(Cl + CCl2CF2) = (6.3 ± 1.5) × 10−11 cm3 per molecule per s. These rate coefficients were compared with previous literature data to analyze the effect of halogen substitution in ethenes on the reactivity towards OH and Cl, and used to estimate the global atmospheric lifetimes for the studied haloethenes. The calculated lifetimes, using average global concentrations of OH radicals and Cl atoms, indicate that the atmospheric loss of these compounds is determined by the OH-initiated oxidation. Also, the atmospheric implications of the halogenated ethenes studied were evaluated by estimating acceptability indices such as the global warming potential (GWP) and the ozone depletion potential (ODP). From these potentials, the contribution of (E/Z)-CHClCHF, (E/Z)-CFClCFCl, and CCl2CF2 to radiative forcing of climate change and to ozone layer depletion is expected to be negligible.