H Atom Yields from the Reactions of CN Radicals with C2H2, C2H4, C3H6, trans-2-C4H8, and iso-C4H8

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2007-07, Vol.111 (29), p.6679-6692
• Main Authors: Gannon, Kelly L, Glowacki, David R, Blitz, Mark A, Hughes, Kevin J, Pilling, Michael J, Seakins, Paul W
• Format: Article
• Language: English
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp0689520

The kinetics and H atom channel yield at both 298 and 195 K have been determined for reactions of CN radicals with C2H2 (1.00 ± 0.21, 0.97 ± 0.20), C2H4 (0.96 ± 0.032, 1.04 ± 0.042), C3H6 (pressure dependent), iso-C4H8 (pressure dependent), and trans-2-C4H8 (0.039 ± 0.019, 0.029 ± 0.047) where the first figure in each bracket is the H atom yield at 298 K and the second is that at 195 K. The kinetics of all reactions were studied by monitoring both CN decay and H atom growth by laser-induced fluorescence at 357.7 and 121.6 nm, respectively. The results are in good agreement with previous studies where available. The rate coefficients for the reaction of CN with trans-2-butene and iso-butene have been measured at 298 and 195 K for the first time, and the rate coefficients are as follows:  k 298K = (2.93 ± 0.23) × 10-10 cm3 molecule-1 s-1, k 195K = (3.58 ± 0.43) × 10-10 cm3 molecule-1 s-1 and k 298K = (3.17 ± 0.10) × 10-10 cm3 molecule-1 s-1, k 195K = (4.32 ± 0.35) × 10-10 cm3 molecule-1 s-1, respectively, where the errors represent a combination of statistical uncertainty (2σ) and an estimate of possible systematic errors. A potential energy surface for the CN + C3H6 reaction has been constructed using G3X//UB3LYP electronic structure calculations identifying a number of reaction channels leading to either H, CH3, or HCN elimination following the formation of initial addition complexes. Results from the potential energy surface calculations have been used to run master equation calculations with the ratio of primary:secondary addition, the average amount of downward energy transferred in a collision 〈ΔE d〉, and the difference in barrier heights between H atom elimination and an H atom 1, 2 migration as variable parameters. Excellent agreement is obtained with the experimental 298 K H atom yields with the following parameter values:  secondary addition complex formation equal to 80%, 〈ΔE d〉 = 145 cm-1, and the barrier height for H atom elimination set 5 kJ mol-1 lower than the barrier for migration. Finally, very low temperature master equation simulations using the best fit parameters have been carried out in an increased precision environment utilizing quad−double and double−double arithmetic to predict H and CH3 yields for the CN + C3H6 reaction at temperatures and pressures relevant to Titan. The H and CH3 yields predicted by the master equation have been parametrized in a simple equation for use in modeling.