Absolute rate constants for elementary reactions in the chlorination of CH4, CD4, CH3Cl, CH2Cl2, CHCl3, CDCl3 and CBrCl3

Abstract

The rates of reactions of Cl²P atoms with methane, and with a series of substituted methanes, have been measured using mass spectrometric analysis of molecular reactant consumption in the presence of excess Cl atoms, with pseudo first order kinetic analysis; this approach minimizes secondary reactions and the problem of reaction stoichiometries. The following rate constants were obtained for Cl + RH reactions: log₁₀(k_CH4/cm³ molecule^–1 s^–1)=(–10.294 ± 0.043)–(778 ± 16) K/T, 300–686 K; log₁₀(k_CH3Cl/cm³ molecule^–1 s^–1)=(–9.673 ± 0.087)–(776 ± 30) K/T, 300–604 K; log₁₀(k_CH2Cl2/cm³ molecule^–1 s^–1)=(–10.078 ± 0.074)–(629 ± 26) K/T, 298–621 K; log₁₀(k_CHCl3/cm³ molecule^–1 s^–1)=(–10.480 ± 0.051)–(599 ± 19) K/T, 297–652 K; log₁₀(k_CBrCl3/cm³ molecule^–1 s^–1)=–12.64 ± 0.08, 652 K. The correlations of these results with data from competitive chlorination studies are discussed in relation to the uncertainly surrounding the Arrhenius parameters for the primary reference reaction, Cl + H₂→ HCl + H.

Values for the kinetic isotope (H/D) rate constant ratios, (k_CH4/k_CD4) and (k_CHCl3/k_CDCl3), were measured over wide ranges of temperature: log₁₀(k_CH4/k_CD4)=(–0.155 ± 0.024)+(384 ± 12) K/T, 298–1018 K; log₁₀(k_CHCl3/k_CDCl3)=(–0.014 ± 0.020)+(167 + 9) K/T, 298–1006 K. These results are in agreement with the predictions of the semi-empirical BEBO (bond-energy-bond order) method.