Tropospheric Oxidation of Ethyne and But-2-yne. 1. Theoretical Mechanistic Study

Abstract

This paper (part 1) and the following one (part 2) aim to assess the viability of some tropospheric oxidation channels for two symmetrical alkynes, ethyne (acetylene) and but-2-yne. Paper 1 defines the features of the DFT(B3LYP)/6-311G(3df,2p) energy hypersurface and qualitatively considers the practicability of different pathways through the estimate of free energy barriers. Paper 2 will assess this in more detail by way of master equation simulations. Oxidized in the presence of HO and O2 (with the possible intervention of NO), ethyne and but-2-yne are known to produce mainly glyoxal or dimethylglyoxal and, to a lesser extent, formic or acetic acid. The initial attack by HO gives an adduct, from which several pathways (1a-c, 2a-e) originate. Pathway 1a passes through the 2-oxoethyl (vinoxyl) radical, or the analogous dimethyl-substituted intermediate, which could in principle undergo O2 addition (and subsequently, but through a demanding step, give the dialdehydes). However, in paper 2 it is assessed that the vinoxyl, as a nonthermalized intermediate, will preferentially follow unimolecular pathways to ketene or acetyl. Pathway 2a is the most important pathway: a very steep free energy cascade, started by O2 addition to the initial HO adduct with a concerted barrierless 1,5 H shift, gives a hydroperoxyalkenyloxyl radical intermediate. Peroxy bond cleavage finally produces the dialdehydes and regenerates HO. Pathways 2b and 2c originate from O2 addition to the initial HO adduct and produce, via different ring closures, either dioxetanyl or alkyl dioxiranyl radicals, respectively. Two subsequent fragmentations occur in both cases and give the carboxylic acids and a carbonyl radical, which can indirectly generate hydroxyl. Two further pathways (1c and 2e) see NO intervention onto the peroxyl radicals formed along pathways 1 and 2. Both could enhance dialdehyde production, while simultaneously depressing the carboxylic acid yield.

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 [41,[77][78][79][80][81][82][83][84][85]. Reported data on the branching fraction at low temperature are in good agreement.…”

“…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,…”

“…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 [41,[76][77][78][79][80][81][82][83][84][85]. Hatakeyama et al [76] showed in smoke-chamber experiments that the reaction generates glyoxal and formic acid,…”

Atmospheric reactivity of HCCCH2OH (2-propyn-1-ol) toward OH radicals: experimental determination and theoretical comparison with its alkyne analogue

“…[6][7][8] While there are several studies concerning the degradation of unsaturated alcohols initiated by the main tropospheric oxidants, [9][10][11][12] information on the reactions of alkynols is still scarce. 15 Calvert et al 16 have suggested that the value obtained by Upadhyaya et al for OH + 2P1OL at low pressure is not applicable to atmospheric conditions, and tentatively ascribed those results to a pressure dependence of the rate coefficient like in the reaction of allyl alcohol + OH radicals. In this latter study, the authors evaluated also the possibility of formation of different adducts depending on the position of the OH radical attack to the C-C triple bond.…”