Dariusz Sarzyński

Atmospheric Chemistry of 2,3-Pentanedione: Photolysis and Reaction with OH Radicals

The kinetics of the overall reaction between OH radicals and 2,3-pentanedione (1) were studied using both direct and relative kinetic methods at laboratory temperature. The low pressure fast discharge flow experiments coupled with resonance fluorescence detection of OH provided the direct rate coefficient of (2.25 ± 0.44) × 10(-12) cm(3) molecule(-1) s(-1). The relative-rate experiments were carried out both in a collapsible Teflon chamber and a Pyrex reactor in two laboratories using different reference reactions to provide the rate coefficients of 1.95 ± 0.27, 1.95 ± 0.34, and 2.06 ± 0.34, all given in 10(-12) cm(3) molecule(-1) s(-1). The recommended value is the nonweighted average of the four determinations: k(1) (300 K) = (2.09 ± 0.38) × 10(-12) cm(3) molecule(-1) s(-1), given with 2σ accuracy. Absorption cross sections for 2,3-pentanedione were determined: the spectrum is characterized by two wide absorption bands between 220 and 450 nm. Pulsed laser photolysis at 351 nm was used and the depletion of 2,3-pentanedione (2) was measured by GC to determine the photolysis quantum yield of Φ(2) = 0.11 ± 0.02(2σ) at 300 K and 1000 mbar synthetic air. An upper limit was estimated for the effective quantum yield of 2,3-pentanedione applying fluorescent lamps with peak wavelength of 312 nm. Relationships between molecular structure and OH reactivity, as well as the atmospheric fate of 2,3-pentanedione, have been discussed.

Laser spectrometry and kinetics of selected elementary reactions of the acetonyl radical

The laser induced fluorescence excitation spectrum (LIF) and the ultraviolet absorption spectrum (TA) of the acetonyl radical (CH3C(O)CH2) were remeasured by using the time-resolved fast discharge flow (DF) and laser flash photolysis (LP) experimental techniques (T = 298 K). The absorption cross section of σ(acetonyl, 207 nm) = (3.16 ± 0.61) × 10−18 cm2 molecule−1 was determined calibrated against the acetyl-peroxyl radical (CH3C(O)O2) in LP/TA measurements. The kinetics of the reactions of CH3C(O)CH2 with the open shell reaction partners O2 (1), NO (2), NO2 (3) and H (4) were studied by using the DF method with LIF detection of the acetonyl radical at 298 ± 1 K and 2.85 ± 0.05 mbar He pressure. The rate constants for the overall reactions were determined in units of cm3 molecule−1 s−1 to be k1 = (3.49 ± 0.51) × 10−13, k2 = (1.04 ± 0.19) × 10−11, k3 = (3.25 ± 0.65) × 10−11 and k4 ≥ 3 × 10−10 with 2σ accuracy given. The acetonyl radical was found to react similarly to alkyl radicals by comparison with literature results. A reduced reactivity was observed toward O2 and NO that might be attributed to the resonance stabilisation of the acetonyl radical. No such effect was observed for the NO2 and H atom reactions.

Mechanism of the gas-phase decomposition of trifluoro-, trichloro-, and tribromomethanols in the presence of hydrogen halides

Ab initio calculations at the G2 level were used in a theoretical analysis of the kinetics of the decomposition of trifluoro-, trichloro-, and tribromomethanols. The high-pressure limiting rate coefficients kdiss,∞ for the thermal dissociation of CF3OH, CCl3OH, and CBr3OH were calculated using the conventional transition state theory. The results of potential surface calculations show that in the presence of the hydrogen halides HX (X = F, Cl, and Br), considerably lower energy pathways are accessible for the decomposition of CF3OH, CCl3OH, and CBr3OH. The mechanism of the reactions appears to be complex and consists of three consecutive elementary processes with the formation of pre- and post-reaction adducts. The presence of hydrogen halides considerably decreases the energy barrier for the bimolecular decomposition of the alcohols CF3OH, CCl3OH, and CBr3OH. Results of this study indicate that hydrogen halides can considerably accelerate the homogeneous decomposition of perhalogenated methanols when they are present in the reaction area at sufficiently high concentrations. However, the atmospheric concentrations of hydrogen halides are too small for efficient removal of atmospheric CF3OH, CCl3OH, and CBr3OH.

Experimental and Theoretical Study on the OH-Reaction Kinetics and Photochemistry of Acetyl Fluoride (CH3C(O)F), an Atmospheric Degradation Intermediate of HFC-161 (C2H5F)

The direct reaction kinetic method of low pressure fast discharge flow (DF) with resonance fluorescence monitoring of OH (RF) has been applied to determine rate coefficients for the overall reactions OH + C2H5F (EtF) (1) and OH + CH3C(O)F (AcF) (2). Acetyl fluoride reacts slowly with the hydroxyl radical, the rate coefficient at laboratory temperature is k2(300 K) = (0.74 ± 0.05) × 10(-14) cm(3) molecule(-1) s(-1) (given with 2σ statistical uncertainty). The temperature dependence of the reaction does not obey the Arrhenius law and it is described well by the two-exponential rate expression of k2(300-410 K) = 3.60 × 10(-3) exp(-10500/T) + 1.56 × 10(-13) exp(-910/T) cm(3) molecule(-1) s(-1). The rate coefficient of k1 = (1.90 ± 0.19) × 10(-13) cm(3) molecule(-1) s(-1) has been determined for the EtF-reaction at room temperature (T = 298 K). Microscopic mechanisms for the OH + CH3C(O)F reaction have also been studied theoretically using the ab initio CBS-QB3 and G4 methods. Variational transition state theory was employed to obtain rate coefficients for the OH + CH3C(O)F reaction as a function of temperature on the basis of the ab initio data. The calculated rate coefficients are in good agreement with the experimental data. It is revealed that the reaction takes place predominantly via the indirect H-abstraction mechanism involving H-bonded prereactive complexes and forming the nascent products of H2O and the CH2CFO radical. The non-Arrhenius behavior of the rate coefficient at temperatures below 500 K is ascribed to the significant tunneling effect of the in-the-plane H-abstraction dynamic bottleneck. The production of FC(O)OH + CH3 via the addition/elimination mechanism is hardly competitive due to the significant barriers along the reaction routes. Photochemical experiments of AcF were performed at 248 nm by using exciplex lasers. The total photodissociation quantum yield for CH3C(O)F has been found significantly less than unity; among the primary photochemical processes, C-C bond cleavage is by far dominating compared with CO-elimination. The absorption spectrum of AcF has also been determined by displaying a strong blue shift compared with the spectra of aliphatic carbonyls. Consequences of the results on atmospheric chemistry have been discussed.

Theoretical study of the kinetics of reactions of the monohalogenated methanes with atomic chlorine

Ab initio calculations at the G2 level were used in a theoretical description of the kinetics and mechanism of the hydrogen abstraction reactions from fluoro-, chloro- and bromomethane by chlorine atoms. The profiles of the potential energy surfaces show that mechanism of the reactions under investigation is complex and consists of two - in the case of CH3F+Cl - and of three elementary steps for CH3Cl+Cl and CH3Br+Cl. The heights of the energy barrier related to the H-abstraction are of 8–10 kJ mol−1, the lowest value corresponds to CH3Cl+Cl and the highest one to CH3F+Cl. The rate constants were calculated using the theoretical method based on the RRKM theory and the simplified version of the statistical adiabatic channel model. The kinetic equations derived in this study

Competitive bromination kinetics of CH3Br and CH2ClBr

\begin{array}{*{20}c} {k\left( {\mathrm{C}{{\mathrm{H}}_3}\mathrm{F}+\mathrm{Cl}} \right)=6.75\times 1{0^{-12 }}\times {{{\left( {\mathrm{T}/300} \right)}}^{2.12 }}\times\exp (-900/\mathrm{T})}{\mathrm{c}{{\mathrm{m}}^3}\mathrm{molecul}{{\mathrm{e}}^{-1 }}{{\mathrm{s}}^{-1 }}} \\ {k\left( {\mathrm{C}{{\mathrm{H}}_3}\mathrm{Cl}+\mathrm{Cl}} \right)=6.97\times 1{0^{-12 }}\times {{{\left( {\mathrm{T}/300} \right)}}^{1.73 }}\times\exp (-795/\mathrm{T})}{\mathrm{c}{{\mathrm{m}}^3}\mathrm{molecul}{{\mathrm{e}}^{-1 }}{{\mathrm{s}}^{-1 }}} \\ {k\left( {\mathrm{C}{{\mathrm{H}}_3}\mathrm{Br}+\mathrm{Cl}} \right)=6.26\times 1{0^{-12 }}\times {{{\left( {\mathrm{T}/300} \right)}}^{1.82 }}\times\exp (-795/\mathrm{T})}{\mathrm{c}{{\mathrm{m}}^3}\mathrm{molecul}{{\mathrm{e}}^{-1 }}{{\mathrm{s}}^{-1 }}} \\ \end{array}

Gas-phase reactions of Cl atoms with propane, n-butane, and isobutane

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