Kinetics of the reaction Cl + CH₄ → CH₃+ HCl from 200° to 500deg;K

Abstract

The rate constant for the reaction C1+ CH, 1 , CH3 + HCl has been determined over the temperature range of 200"-500"K using a discharge flow system with resonance fluorescence detection of atomic chlorine under conditions of large excess CH4. For 300" > 7' > 200°K the data are best fitted to the expression kl = (8.Curvature is observed in the Arrhenius plot such that the effective activation energy increases from 2.6 kcal/mol at 200" < T < 300°K to 3.5 kcal/mol at 360" < T < 500°K. The data over the entire range may be fitted by the expression k l = 8.6 X T2." exp[-795/T]. These results are compared with other experimental studies and with a semiempirical transition state calculation. Their atmospheric significance is discussed.

Reduced dimensionality spin-orbit dynamics of CH3 + HCl $\rightleftharpoons$⇌ CH4 + Cl on ab initio surfaces

“…Thermal rate constants for the reverse reaction were calculated for the IB and Q//T surfaces and compared to experimental [97][98][99][100] and evaluated 95 kinetic data, as well as previous theoretical results 7,11,13 (Figure 5b). Using the highest level of ab initio theory to compute the surfaces (IB) resulted in thermal rate constants in excellent agreement with experimental results, though slightly larger than experiment at low temperatures.…”

Quasi-classical trajectory study of the dynamics of the Cl + CH4→ HCl + CH3 reaction

“…The barrier height is within 0.7 kJ mol -1 of best high level calculations and the activation energy from Arrhenius analysis of the temperature dependence of the rate coefficients. 39 The computed geometry of the SRP-AM1 TS, shown in Table II, has bond lengths within 0.02 Å, and angles within 1.1° of CCSD(T) and QCISD-SAC calculations. Table III Despite the relatively shallow depth (13.24 kJ mol -1 below the TS and 7.95 kJ mol -1 below the products), this minimum may be significant in the reaction dynamics as the excess collision energy over the barrier employed in the calculations here is only ~4 kJ mol -1 .…”

The temperature coefficient of the rates in the system Cl + CH4 ⇆ CH3 + HCl, thermochemistry of the methyl radical

“…It has also been proposed that quantum mechanical tunneling occurs to account for this curvature [5,17]. In light of the low (-0.5 kcal) intrinsic activation energy, we think this contribution would be negligible.…”