Rate Constant for the Recombination Reaction CH₃ + CH₃ → C₂H₆ at T = 298 and 202 K

Abstract

The recombination of methyl radicals is the major loss process for methyl in the atmospheres of Saturn and Neptune. The serious disagreement between observed and calculated levels of CH has led to suggestions that the atmospheric models greatly underestimated the loss of CH due to poor knowledge of the rate of the reaction CH + CH + M → CH + M at the low temperatures and pressures of these atmospheric systems. In an attempt to resolve this problem, the absolute rate constant for the self-reaction of CH has been measured using the discharge-flow kinetic technique coupled to mass spectrometric detection at T = 202 and 298 K and P = 0.6-2.0 Torr nominal pressure (He). CH was produced by the reaction of F with CH, with [CH] in large excess over [F], and detected by low energy (11 eV) electron impact ionization at m/ z = 15. The results were obtained by graphical analysis of plots of the reciprocal of the CH signal vs reaction time. At T = 298 K, k (0.6 Torr) = (2.15 ± 0.42) × 10 cm molecule s and k (1 Torr) = (2.44 ± 0.52) × 10 cm molecule s. At T = 202 K, the rate constant increased from k (0.6 Torr) = (5.04 ± 1.15) × 10 cm molecule s to k (1.0 Torr) = (5.25 ± 1.43) × 10 cm molecule s to k (2.0 Torr) = (6.52 ± 1.54) × 10 cm molecule s, indicating that the reaction is in the falloff region. Klippenstein and Harding had previously calculated rate constant falloff curves for this self-reaction in Ar buffer gas. Transforming these results for a He buffer gas suggest little change in the energy removal per collision, -〈Δ E〉, with decreasing temperature and also indicate that -〈Δ E〉 for He buffer gas is approximately half of that for Argon. Since the experimental results seem to at least partially affirm the validity of the Klippenstein and Harding calculations, we suggest that, in atmospheric models of the outer planets, use of the theoretical results for k is preferable to extrapolation of laboratory data to pressures and temperatures well beyond the range of the experiments.

Photochemistry of C3Hphydrocarbons in Titan’s stratosphere revisited

“…Units on the left axis are in cm 6 molecule −2 s −1 for k 0 and cm 3 molecule −1 s −1 for k r . Experimental data are taken from: H + NH 2 (Gordon et al 1971), H + CH 3 (Brouard et al 1989), CH 3 + O 2 (Fernandes et al 2006), CH 3 + NO (Kaiser 1993), CH 3 + NH 2 (Jodkowski et al 1995), CH 3 + CH 3 (Slagle et al 1988;Cody et al 2002). The solid and dashed lines correspond to the general expression we derived for the low-pressure limiting rate constants k 0 for H addition and CH 3 addition, respectively.…”

“…We also neglected adduct stabilization. This reaction has been widely studied (Slagle et al 1988;Cody et al 2002Cody et al , 2003. Wang et al (2003) have fitted various experimental data between 202 and 900 K, leading to k 0 (T ) = 1.14 × 10 −25 × (T/300) −3.75 × exp (−494/T ) cm 6 molecule −2 s −1 and k ∞ (T ) = 7.4 × 10 −11 × (T/300) −0.69 × exp(−88/T ) cm 3 molecule −1 s −1 .…”

On the Reaction Kinetics of Chemically Active Molecular Microwave Plasmas

“…The rate constant k 1 for the self recombination reaction of methyl radicals has been extensively investigated in experimental and theoretical works [87][88][89][90][91][92][93][94]. The chosen value for k 1 has been obtained on the base of the compilation of Baulch et al and was adapted to the specific temperature and argon concentration [95,96].…”

Kinetic and Diagnostic Studies of Molecular Plasmas Using Laser Absorption Techniques

“…The rate constant k 1 for the self recombination reaction of methyl radicals has been extensively investigated in experimental and theoretical work [31][32][33][34][35][36][37][38]. The selected value for k 1 was based on the compilation of Baulch et al and was appropriate for the specific temperature and argon concentration [28,39].…”