Emil Ratajczak

KINETICS OF THE GAS PHASE REACTION OH + NO(+ M) HONO(+ M) AND THE DETERMINATION OF THE UV ABSORPTION CROSS SECTIONS OF HONO

Abstract The reaction OH + NO(+M) → HONO(+M) with M = SF 6 as a third body has been employed as a clean source for recording the near-ultraviolet absorption spectrum of HONO without interference from other absorbing species. The reaction was initiated by the pulse radiolysis of SF 6 /H 2 O/NO mixtures with total pressures in the range 10–1000 mbar at 298 K. The pressure dependence of the rate coefficient was studied by time-resolved UV and IR spectroscopy. By analysis of the fall-off curve we have derived a value for the limiting low pressure rate constant k 0 /[SF 6 ] = (1.5 ± 0.1) × 10 −30 cm 6 molecule −2 s −1 at 298 K, using the values of k ∞ = (3.3±0.3)×10 −11 cm 3 molecule −1 s −1 and F cent = 0.81 reported by Troe and co-workers. The UV spectrum of HONO was recorded in the range 320–400 nm and an absolute absorption cross section of σ = (5.02 ± 0.76) × 10 −19 cm 2 molecule −1 has been determined for the strongest band of HONO located at 354.2 nm. Differential absorption cross sections to be used for field measurements of HONO were also investigated.

Kinetics of the reactions of CH2Br and CH2I radicals with molecular oxygen at atmospheric temperatures.

The kinetics of the reactions of CH2Br and CH2I radicals with O2 have been studied in direct measurements using a tubular flow reactor coupled to a photoionization mass spectrometer. The radicals have been homogeneously generated by pulsed laser photolysis of appropriate precursors at 193 or 248 nm. Decays of radical concentrations have been monitored in time-resolved measurements to obtain the reaction rate coefficients under pseudo-first-order conditions with the amount of O2 being in large excess over radical concentrations. No buffer gas density dependence was observed for the CH2I + O2 reaction in the range 0.2-15 x 10(17) cm(-3) of He at 298 K. In this same density range the CH2Br + O2 reaction was obtained to be in the third-body and fall-off area. Measured bimolecular rate coefficient of the CH2I + O2 reaction is found to depend on temperature as k(CH2I + O2)=(1.39 +/- 0.01)x 10(-12)(T/300 K)(-1.55 +/- 0.06) cm3 s(-1)(220-450 K). Obtained primary products of this reaction are I atom and IO radical and the yield of I-atom is significant. The rate coefficient and temperature dependence of the CH2Br + O2 reaction in the third-body region is k(CH2Br + O2+ He)=(1.2 +/- 0.2)x 10(-30)(T/300 K)(-4.8 +/- 0.3) cm6 s(-1)(241-363 K), which was obtained by fitting the complete data set simultaneously to a Troe expression with the F(cent) value of 0.4. Estimated overall uncertainties in the measured reaction rate coefficients are about +/-25%.

Ultraviolet absorption spectra and kinetics of acetonyl and acetonylperoxy radicals

Abstract Acetonyl radicals and acetonylperoxy radicals were produced by pulse radiolysis of gas mixtures with varying concentrations of CH3COCH3 and O2 in 1 atm of SF6 to initiate the reactions (1) F+CH3COCH3→HF+CH3COCH2, (2) 2CH3COCH2 → products, (3) CH3COCH2+O2(+M) → CH3COCH2O2(+M) and (4) 2CH3COCH2O2 → products. The ultraviolet absorption spectrum of the acetonyl radical is composed of a fairly weak band centered at 315 nm and a stronger band in the range of 200–250 nm. The absorption cross section has been determined, σ(310 nm) = (8.73 ± 0.50) × 10−19 cm2 molecule−1. The values k2=(4.8 ± 0.4) × 10−11, k3=(1.5 ± 0.3) × 10−12 and k4=(8.3 ± 1.6) × 10−12 cm3 molecule−1 s−1 at 298 K were derived by computer modelling of the observed kinetic features.

Kinetics of the reactions H + C2H4 → C2H5, H + C2H5 → 2CH3 and CH3 + C2H5 → products studied by pulse radiolysis combined with infrared diode laser spectroscopy

Abstract Formation of methyl radicals via the consecutive reactions H + C 2 H 4 + M → C 2 H 5 + M (1) and H + C 2 H 5 → CH 3 + CH 3 (2a) was initiated by pulse radiolysis of 10–100 mbar H 2 in the presence of ethylene. The kinetics of CH 3 were studied by monitoring the transient infrared absorption at the Q(3, 3) line of the ν 2 = 0 → 1 vibrational transition at 606.12 cm −1 . Reaction (2a) proceeds in competition with the reaction H + C 2 H 5 + M → C 2 H 6 + M (2b). The methyl radicals are consumed by CH 3 + CH 3 + M → C 2 H 6 + M (3), CH 3 + C 2 H 5 → products (4), and H + CH 3 + M → CH 4 + M (5). By computer simulations based on determined absolute radical yields, the observed formation and decay kinetics could be reproduced using the rate constants k 2a = (7.5 ± 0.7) X 10 10 M −1 s −1 , k 2b = (1.0 ± 0.1) X 10 11 M −1 s −1 , and k 4 = (4.0 ± 0.2) X 10 10 M −1 s −1 at 298 K and a total pressure of p (H 2 ) = 100 mbar.

Kinetics of the self-reaction and the reaction with OH of the amidogen radical

Abstract The kinetics of the self-reaction and the reaction with OH of the amidogen radical were studied at 298 K by pulse radiolysis combined with UV detection of NH 2 and OH radicals at 597.7 and 309.0 nm. The rate constant for the association reaction of NH 2 radicals was found to be pressure dependent in the range 200–1000 mbar of SF 6 , and by Troes analysis the following high- and low-pressure limiting rate constants were obtained: κ rec, ∞ (7.1 ± .8) × 10 10 ( T /300) 0.27 M −1 s −1 and κ rec, 0 /[SF 6 ]=(7.1 ± 1.5) × 10 12 ( T /300) −3.9 M −2 s −1 in the temperature range 200–400 K. For the reaction between NH 2 and OH radicals no pressure dependence was observed in the range 500–1000 mbar SF 6 , and the following high-pressure limiting rate constant was derived: (5.6 ± 1.0) × 10 10 ( T /300) 0.20 M −1 s −1 in the temperature range 200–400 K.

Spectrokinetic studies of the gas-phase equilibrium F+O2⇌FO2 between 295 and 359 K

Abstract Spectrokinetic studies of the gas-phase equilibrium F+O 2 ⇌FO 2 have been undertaken at six temperatures between 295 and 359 K. F atoms were produced by pulse radiolysis of Ar/F 2 /O 2 mixtures. The yield of FO 2 radicals and the rates of formation and decay were studied as a function of the oxygen concentration, and the temperature dependences of both the forward and reverse rate constants, k 1 and k −1 , were obtained. The equilibrium constant, K p = k 1 / k −1 RT , was shown to be pressure independent. The computed value of Δ S 298 0 = −24.917 cal/mol K, when combined with the other calculated thermodynamic variables and the measured equilibrium constants, gives Δ H 298 0 = −12.62±0.50 kcal/mol, Δ H f,298 0 (FO 2 )=6.24±0.50 kcal/mol and an intrinsic FO 2 bond strength Δ H 0 0 =11.68±0.50 kcal/mol.

Kinetics of the gas-phase reaction between ethyl and hydroxyl radicals

The kinetics of the reactions C2H5+OH(+M)→products (+M) (1) were studied at room temperature by pulse radiolysis of C2H6/H2O/SF6 mixtures. The ethyl radical decay was followed by monitoring the UV absorption signals at 205 nm. The experimentally determined value of k1 is (7.1 ± 1.0) × 1010 M−1 s−1. No pressure dependence of k1 in the pressure range 250–1000 mbar SF6 was observed. The calculated k1a,∞ value for the temperature range 200–400 K is (7.7 ± 12.0) × 1010 M−1 s−1 for the addition reaction. The low-pressure limiting rate constant for the addition reaction should not be lower than 1017 M−2 s−1 at room temperature.

Kinetics of the cross reaction between methyl and hydroxyl radicals

Abstract The kinetics of the reaction CH 3 +OH (+M)→CH 3 OH (+M) was studied at room temperature by pulse radiolysis of SF 6 /CH 4 /H 2 O mixtures. The methyl radical decay was followed by monitoring the UV absorption signals at 216.4 nm. A pressure dependence of k 1 was observed with values ranging from (5.8±0.3)×10 10 M −1 s −1 at 85 mbar to (7.8±0.7)×10 10 M −1 s −1 at 1000 mbar SF 6 . The high- and low-pressure limiting rate constants were calculated. k 1,∞ is for the temperature range 200–500 K expressed as (8.7±0.9)×10 10 ×( T /300) 0.1 M −1 s −1 . The low-pressure limiting rate constant is estimated to be k 1,0 /[SF 6 ] = (2.6±2.3)×10 15 M −2 s −1 at room temperature.

Pressure and temperature dependence of the gas-phase reaction between methyl and hydroxyl radicals

Abstract The kinetics of the reaction CH 3 + OH (+M)→CH 3 OH(+M) was studied in the temperature range 283–373 K and in the pressure range 85–1000 mbar SF 6 by pulse radiolysis combined with UV absorption spectroscopy. The methyl and hydroxyl radical decays were followed by monitoring the transient UV absorption signals at 216.4 nm and 309.0 nm respectively. The reaction was studied in the fall-off region and by Troes analysis the following high- and low-pressure limiting rate constants were obtained: k rec,∞ =(8.7±0.7)×10 10 ( T /300) 0.1 M −1 s −1 and k rec,0 /[SF 6 ]=(9.0±2.4)×10 14 ( T /300) −3.8 M −2 s −1 . The low-pressure limiting rate constant for the OH radical self-reaction was also derived: k OH+OH,0 /[SF 6 ]=(4.0±0.7)×10 11 ( T /300) −1.4 M −2 s −1 using the value of k OH+OH,∞ =1.8×10 10 ( T /300) −0.37 M −1 s −1 for the high-pressure limit.

Spectrokinetic studies of i-C3H7 and i-C3H7O2 radicals

Abstract UV absorption spectra of i-C3H7 and i-C3H7O2 radicals have been obtained following pulse radiolysis of H2/C3H6 and H2/C3H6/O2 mixtures at a total pressure of 1 atm at room temperature. The i-C3H7 spectrum shows two absorption maxima with ϵ(233 nm) = 890 ± 130 M−1 cm−1 and ϵ(205 nm) = 780 ± 120 M−1 cm−1, respectively. The spectrum of i-C3H7O2 is a broad continuum with a maximum of ϵ=1300±200 M−1 cm−1 at 223 nm. By computer-aided analysis of the kinetic features a consistent set of rate constants for the reactions involved in these systems have been deduced.