You are here: Home :: Area NEM :: Chemistry :: Physical chemistry

J. Deppe, H.Gg. Wagner

Detection of ¹CH₂ Radicals in Hydrocarbon Pyrolysis Behind Shock Waves Using FM Spectroscopy

Singlet methylene radical (¹CH₂) concentrations were measured for the first time in the pyrolysis of methane (CH₄) and ethane (C₂H₆) behind shock waves. The very sensitive frequency modulation (FM) spectroscopy, already established for sensitve detection of amino radicals (NH₂) [1,2], was used for that purpose. Applying computer simulations using a complex reaction mechanism the experimental ¹CH₂ signals were fitted and rate coefficients of different reaction channels were obtained. For the reaction channel

(1a) CH₄ + M → CH₃ + H + M

an extented Arrhenius expression of

k_1a = 6.5 ·10¹⁸ · (T/298)^−1.70 · exp[−366 kJ mol⁻¹/RT] cm³ mol⁻¹ s⁻¹ (± 50%),

and futhermore rate coefficients for

(7) ³CH₂ + CH₃→ H + C₂H₄ with k₇ = 3.2 · 10¹³ cm³ mol⁻¹ s⁻¹ (± 40%),
(11) ³CH₂ + H → CH + H₂ with k₁₁ = 7.9 · 10¹³ cm³ mol⁻¹ s⁻¹ (± 40%),

and for the intersystem crossing via

(6) ¹CH₂ + M → ³CH₂ + M with k₆ = 1.40 ·10¹⁰ · (T/K)^0.9cm³ mol⁻¹ s⁻¹ (± 40%)

were determined.
The experimental conditions ranged from 1900 to 4000 K with corresponding pressures between 0.23 to 0.54 bar.

The ¹CH₂ absorption coefficients were extracted from the numerical evaluation of the experiments as function of temperature. The following (fitted) expression

α(T) = 5.70 · 10⁵ · exp(1.064 · 10⁻³(1690−T/K)) cm²/mol

reflects well the experimental results obtained for the b˜¹B₁(0, 14, 0)4₀₄ ← ã¹A₁(0, 0, 0)4₁₄ transition at 590.707 nm probed in this work.

Zeitschrift für Physikalische Chemie, Oldenbourg Wissenschaftsverlag

Print ISSN: 0942-9352
Volume: 215, 12/2001
Pages: 1501

Journal homepage (external site)

Show all available items of this journal

 

Area NEM