Oxidation of Ethyne and But-2-yne. 2. Master Equation Simulations

Abstract

The aim of this study is to improve understanding of the tropospheric oxidation of ethyne (acetylene, C2H2) and but-2-yne, which takes place in the presence of HO and O2. The details of the potential energy hypersurface have been discussed in a previous article [Maranzana et al., J. Phys. Chem. A 2008, 112, XXXX]. For both molecules, the initial addition of HO radical to the triple bond is followed by addition of O2 to form peroxyl radicals. In both reaction systems, the peroxyl radicals take two isomeric forms, E1 and E2 for ethyne and e1 and e2 for but-2-yne. Energy transfer parameters (alpha = 250 cm-1) for the ethyne system were obtained by simulating laboratory data for N2 buffer gas, where O2 was not present. In simulations of C2H2 + HO when O2 is present, E1 reacts completely and E2 reacts almost completely, before thermalization. Radical E1 produces formic acid ( approximately 44%) and E2 gives glyoxal ( approximately 53%), in quite good agreement with experiments. For but-2-yne, pressure-dependent laboratory data are too scarce to obtain energy transfer parameters directly, so simulations were carried out for a range of values: alpha = 200-900 cm-1. Excellent agreement with the available experimental yields at atmospheric pressure was obtained with alpha = 900 cm-1. Two reaction channels are responsible for acetic acid formation, but one is clearly dominant. Biacetyl is produced by reactions of e1 and, to a minor extent, e2. The peroxyl radical e2 leads to less than 8% of all products. Vinoxyl radical (which has been reported in experiments involving C2H2 + HO) and products of its reactions are predicted to be negligible under atmospheric conditions.

Unimolecular Reactions of Peroxy Radicals in Atmospheric Chemistry and Combustion

“…The lowest-energy route available to T1 involves isomerisation via TS1c(T) to give Int1c(T), a three-membered ring, which then dissociates via TS2c(T) to give HCOOH + HCO. Maranzana et al [25] carried out master equation calculations on this reaction which gave product yields in good agreement with experiment.…”

Experimental and Kinetic Modeling Study of C2H2 Oxidation at High Pressure

“…This demonstrates that the formation of products occurs directly from the peroxy radical. The finding that HOCHCHOO is shortlived and leads to regeneration of the OH radical has been confirmed in a number of experimental and theoretical studies .…”

“…However, the fast reaction with O 2 is the major consumption step, even at higher temperatures. Alzueta et al identified the chain‐propagating sequence as important for the onset of reaction for C 2 H 2 at atmospheric pressure and temperatures above 700 K. Interest in this oxidation pathway for atmospheric chemistry has spawned a range of experimental and theoretical studies on the C 2 H 2 + OH + O 2 reaction at low temperature . Hatakeyama et al showed in smoke‐chamber experiments that the reaction generates glyoxal and formic acid,…”

Gas-phase reaction between calcium monocation and fluoromethane: Analysis of the potential energy hypersurface and kinetics calculations

“…Therefore, despite impressive advances in the accuracy of DFT methodologies accomplished during the last few years, 22 discrepancies between experimentally measured and theoretically predicted rate constants are unavoidable. This important point that have been recently stressed by several authors in different contexts, 47,[54][55][56] should be kept in mind when assessing the output of kinetics theoretical predictions.…”