On the low-temperature limit of HACA

Rates of two mechanisms, hydrogen abstraction C2H2 (carbon) addition (HACA) and C2H2 (carbon) addition hydrogen migration (CAHM), were intercompared with each other at postflame conditions of a laminar premixed flame of ethylene. The analysis showed that HACA is substantially faster than CAHM, in contrast with the conclusions reached in a recent study of Zhang et al. [J. Phys. Chem. A 120 (2016) 683]. The difference between the two studies is largely due to dissimilar assignments of the thermodynamics of the H abstraction and of the islands of stability pulling the HACA reaction sequence. In support of this conclusion, the kinetics and thermodynamics of the key HACA reaction step were recalculated at a reliable level of quantum and reaction-rate theories. The implication of the present results is that both HACA and CAHM mechanisms can explain formation of aliphatic groups chemisorbed at edges of aromatics; however, a quantitative relationship is yet to be established with the experimental observations. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.