Formic acid is another product, possibly from the rearrangement and decomposition of the intermediate II (Hatakeyama et al., 1986).

Siese and Zetzsch (1995) and Bohn and Zetzsch (1998) have studied the OH-C2H2 reaction using FP-RF (see Chapter 5.B.3) and observed biexponential decays of OH. They propose that the adduct I has two channels in its reaction with 02, rather than one as shown above, and that one of the two generates OH and glyoxal, a small portion of which is excited and decomposes to HCO.

The vinoxy radical has also been observed as an intermediate (Schmidt et al., 1985), suggesting that a portion of the initial adduct isomerizes:


Vinoxy radical

The vinoxy radical reacts rapidly with 02 with a high-pressure limiting rate constant /cx = (1.9 + 0.2) X 10"13 cm3 molecule" 1 s"1 at P > 400 Torr (Zhu and Johnston, 1995). While the reaction leads to glyoxal formation, the production of glyoxal is delayed compared to the disappearance of the vinoxy radical, suggesting that a long-lived adduct with 02 is first formed and then subsequently decomposes, in part to glyoxal, ch(0)ch2 + o2 -> [ch(o)ch2oo] -> ->

perhaps along with the regeneration of OH, which Schmidt et al. (1985) also observed in the presence of 02. However, Zhu and Johnston (1995) observed the yield of glyoxal from reaction (59) to be only 15%, whereas Hatakeyama et al. (1986) measured glyoxal yields in the OH-C2H2 reaction of ~70%. Thus reaction (59) cannot be the major path for glyoxal formation in the OH + C2H2 reaction. Another reaction proposed for the vinoxy radical (Gutman and Nelson, 1983) is reaction (60):

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