The Reactions of O(³P) with Alkenes:  The Formyl Radical Channel

Abstract

The HCO product of the reaction of O(3P) with ethene has been detected by cavity ring-down spectroscopy using its A−X transition. For propene a somewhat smaller yield of HCO was obtained but the overall rate constant is much larger. The yield of HCO in this reaction is quite small (∼0.05). Moreover, a large number of other alkenes were tried with negative results. The failure of the 1,2 H atom shift followed by breaking the 1,2 bond implies that the unimolecular decomposition has found a more favorable channel. The proposed mechanism is as follows. For an alkene of the form RCH2CHCH2 the first step is attachment of the O(3P) to the terminal carbon atom, C1. Then, intersystem crossing occurs and finally a H atom shifts from C3 to C2 and not from C1 to C2. In this way a molecule of formaldehyde and an alkene shorter by one carbon atom are formed.

A theoretical study on the reaction mechanisms of O(3P)+1‐butene

“…They identified HCO as a reaction product, and believed that in the reaction of O( 3 P) with 1‐butene , the yields of HCO is relatively high. In contrast, Min et al 13 detected the HCO product of the reaction by cavity ring‐down spectroscopy and reported that the product branching fraction should be low since they could not observe HCO. Our computational results in this work are consistent with the experiment of Koda et al 2.…”

“…The reactions of O( 3 P) with olefins have attracted the experimental and theoretical interests for a long time. These reactions are important in the combustion processes and oxidation mechanisms of hydrocarbon 1–10, among which the reaction of O( 3 P)+1‐butene has been investigated experimentally in recent years 11–14.…”

“…As for the reaction of O( 3 P) with 1‐butene, H atom yield was very low and the vinoxy radical CH 2 CHO product yield was 0.23. Bersohn and coworkers 13 detected the HCO product of the O( 3 P) with alkenes reaction by cavity ring‐down spectroscopy. Without observation of the HCO signal in the reaction of O( 3 P) atom with 1‐butene, they supposed that the product branching fraction should be low.…”

Hydrogen atom release from methyl groups of energized molecules

“…A two photon ͑205.1 nm͒ technique for detecting H atoms has been developed by Goldsmith and has been applied in our laboratory to the study of photodissociation. 13,14 The two photon technique excites a H atom in the 1s ground state to a mixture of 3s and 3d states. One can then conveniently observe either a 3d→2p fluorescence at 656 nm or a 2 p →1s fluorescence at 121.6 nm.…”

“…However, previous experiments on methyl bromide using focused 205.1 nm light gave only the modest energy of 128 kJ/mol. 14 An alternative explanation which we prefer is that the energy is derived from the photodissociation of TMA ϩ . The ionization potential of TMA is 7.82 eV and the energy of a 205.1 nm photon is 6.04 eV.…”