Ab initio study on the dynamics of the oxidation reaction of acetylene

The dynamics of the oxidation reaction of an acetylene molecule with an oxygen atom was investigated through an ab initio molecular orbital method. The three reactions (1) C₂H₂+O→HCCO+H, (2) HCCO+H→CH₂+CO and (3) C₂H₂+O→CH₂+CO, reported in reaction schemes, were confirmed as being the main elementary steps related to the acetylene oxidation but were found to be not single elementary steps, with each one forming intermediary species before the proposed products. These intermediaries are expected to decompose always and much faster than they form, so that those three reactions can be treated as elementary steps without problem. Under ideal conditions, only reaction (3) can be expected to proceed, but vibrations of the atoms in a structure formed during the process can cause its decomposition into HCCO+H, and later the so formed HCCO can meet another H atom and react to CH₂+CO. Reactions (1) and (2) are then found to be, respectively, an early and a late step of reaction (3), but each of the three events can occur, depending on how is the collision between the O atom and the vibrating C₂H₂ molecule. The present results are compared with experimental data reported in the literature, and a qualitative agreement with the data of Miller and Bowman can be seen for reactions (1) and (3), and with the data of Warnatz for reaction (2).