Reactions of O(3P) with alkynes: The CO and H atom channels

Abstract

This is the second in a series of papers on the reaction of O(3P) with alkynes in which the internal state distribution of some products are studied. The first paper dealt with acetylene whose two product channels are CO+CH2 and H+HCCO. The present paper deals with the reactions of a series of higher alkynes; however, just the CO release and the H atom release channels were studied. The CO product was rotationally and vibrationally cold in every case. We therefore infer that, except possibly for acetylene, the initial ketocarbene undergoes intersystem crossing to a singlet state and isomerizes to a substituted ketene which then dissociates through a linear C–C–O transition state. The absence of CO vibration energy implies that the energy taken from the initially formed C–O bond to facilitate a 1,2 migration is not returned. The large H atom translational energy implies that the H atom is released simultaneously with the formation of a radical of high resonance energy. Finally, the CO and H atom yields decrease in the longer alkynes, presumably because the dominant reaction channel becomes C–C bond breaking leading to radical pair formation.