Detailed Microvariational RRKM Master Equation Analysis of the Product Distribution of the C₂H₂ + CH(X²Π) Reaction over Extended Temperature and Pressure Ranges

Abstract

We present a detailed microvariational RRKM Master Equation analysis of the CH(X 2 Π) + C 2 H 2 reaction products distribution, based on an earlier quantum chemical characterization of the accessible potential energy surface. This is the first time that the pressure and temperature dependence of the product distribution has been analyzed and discussed. Also, an extensive error analysis is performed, aiming to quantify the confidence region for the predicted product distribution as a function of temperature and pressure. The results indicate that for pressures up to several atmospheres, H + triplet prop-2-enylidene (HCCCH) and H + singlet cyclopropenylidene (cyc-C 3 H 2 ) are the main reaction products, with smaller contributions from C 3 H + H 2 formation. Typical net yields are 90% HCCCH, 7% cyc-C 3 H 2 at 300 K and 82% HCCCH, 11% cyc-C 3 H 2 at 2000 K, for pressures from 0 to 5 atm. Substantial stabilization of 2-propynyl radicals (H 2 CCCH) occurs only at higher pressures in excess of 10 atm of air. Our results disagree with another recent theoretical study, which did not consider several of the most important entrance and exit channels.

Cyclic Versus Linear Isomers Produced by Reaction of the Methylidyne Radical (CH) with Small Unsaturated Hydrocarbons

“…This change is larger than the uncertainties estimated earlier for the B3LYP PES, 17 and decreases the predicted contribution of this exit channel. RRKM-ME calculations on the CH + C 2 H 2 reaction using the CBS-APNO PES find a substantially higher contribution of c-C 3 H 2 (27.0%) versus triplet propargylene (63.5%) compared to the earlier predictions, but still do not indicate the rigid, cyclic C 3 H 2 isomer as the dominant exit channel in the CH + C 2 H 2 reaction (see Table 3).…”

“…17 Within the measured photonenergy range (8.6 eV to 10.8 eV) these three C 3 H 2 isomers are expected to be the only contributors to the ion signal at m/z=38. Franck-Condon factors calculated by Taatjes et al 30 for both C 3 H 2 isomers, the photoionization cross sections at an energy below 9.5 eV are within the same order of magnitude, triplet propargylene having the largest photoionization cross section.…”

“…RRKM Master Equation (RRKM-ME) analysis were then carried out on this CBS-APNO surface; the kinetic methodology was discussed in detail earlier 17 and is described only briefly here. The energy-specific rate coefficients k(E) in the energy-grained ME were calculated using RRKM theory, based on the CBS-APNO//QCISD data, in a rigid rotor harmonic oscillator approximation.…”

“…[17][18][19]43,64 Stabilization of C 3 H 3 intermediates are usually not considered as important exit channels due to the high exothermicity for the reaction intermediate formation. For the entrance channel, insertion of the CH radical into the C−H bond is considered to be unfavorable due to the large change of entropy compared to cycloaddition onto the π-electron system.…”

“…Using RRKM master-equation analysis, the authors provided a complete temperature and pressure dependence for the C 3 H 2 product isomer distribution. 17 They concluded that no matter which entrance channel is dominant, propargyl will be the main reaction intermediate and that any cyclic C 3 H 3 isomers formed are expected to isomerize to the more stable propargyl radical. This calculation also predicts tripletpropargylene as the main reaction product (around 90% at 298 K).…”