Tropospheric oxidation of ethyne and but-2-yne. I. Theoretical mechanistic study

This paper (1) and the following (2) aim to assess the viability of some tropospheric oxidation channels for two symmetrical symmetrical alkines, ethyne (acetylene) and but-2-yne. Paper 1 defines the features of the DFT(B3LYP)/6-31G(3df,2p) energy hypersurface and qualitatively consider the practicability of different pathways through the estimate of free energy barriers. Paper 2 will assess this in more details by way of master equation simulation. Oxidized in the presence of HO and O2 (with possible intervention of NO), ethyne and but-2-yne are known to produce mainly glyoxal or dimethyl glyoxal 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 throught a demanding step, give the dialdehydes). However, in Paper 2 it is assessed that the vinoxyl, as a non-thermalized 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. Pathway 2b and 2c originate from O2 addition to the initial HO adduct and produce, via different ring closure, either dioxetanyl or 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 (1c and 2e) see NO intervention onto the peroxyl radical formed along Pathway 1 and 2. Both could enhance dialdehyede production, while simultaneously depressing the carboxylic acid yield.