Mechanism and rate of the reaction CH3 + O—revisited

Abstract

The primary products and the rate of the reaction of methyl radicals with oxygen atoms in the gas phase at room temperature have been studied using three different experimental arrangements: (A) laser flash photolysis to produce CH₃ and O from the precursors CH₃I and SO₂ (the educts and the products were detected by quantitative FTIR spectroscopy); (B) the coupling of a conventional discharge flow reactor via a molecular sampling system to a mass spectrometer with electron impact ionization, which allowed the determination of labile and stable species; (C) laser induced multiphoton ionization combined with a TOF mass spectrometer–molecular beam sampling–flow reactor, which was used for the specific and sensitive detection of the CH₃, CD₃, C₂H₅ and C₂D₅ radicals and the determination of rate coefficients. The branching ratio of the reaction channels was determined by the experimental arrangements (A) and (B) leading to CH₃ + O → HCHO + H (55 ± 5)% → CO + H₂ + H (45 ± 5)%. The rate coefficients of the normal and deuterated methyl and ethyl radicals with atomic oxygen showed no isotope effect: k(CD₃ + O)/k(CH₃ + O) = 0.99 ± 0.12, k(C₂D₅ + O)/k(C₂H₅ + O) = 1.01 ± 0.07 (statistical error, 95% confidence level). The absolute rate coefficient of the reaction CH₃ + O was derived with reference to the reaction C₂H₅ + O (k = 1.04 × 10¹⁴ cm³ mol⁻¹ s⁻¹) leading to k(CH₃ + O) = (7.6 ± 1.4) × 10¹³ cm³ mol⁻¹ s⁻¹.