Abstract
The dynamics of the interfacial reactions of O((3)P) with the hydrocarbon liquids squalane (C30H62, 2,6,10,15,19,23-hexamethyltetracosane) and squalene (C30H50, trans-2,6,10,15,19,23-hexamethyltetracosa-2,6,10,14,18,22-hexaene) have been studied experimentally. Laser-induced fluorescence (LIF) was used to detect the nascent gas-phase OH products. The O((3)P) atoms are acutely sensitive to the chemical differences of the squalane and squalene surfaces. The larger exothermicity of abstraction from allylic C-H sites in squalene is reflected in markedly hotter OH rotational and vibrational distributions. There is a more modest increase in translational energy release. A larger fraction of the available energy is deposited in the liquid for squalene than for squalane, consistent with a more extensive geometry change on formation of the allylic radical co-product. Although the dominant reaction mechanism is direct, impulsive scattering, there is some evidence for OH being accommodated at both liquid surfaces, resulting in thermalised translation and rotational distributions. Despite the H-abstraction reaction being strongly favoured energetically for squalene, the yield of OH is substantially lower than for squalane. This is very likely due to competitive addition of O((3)P) to the unsaturated sites in squalene, implying that double bonds are extensively exposed at the liquid surface.
Highlights
Chemical processes at gas–liquid interfaces have received increasing attention in recent years
The present study aims to explore how the interfacial dynamics of H abstraction are affected when unsaturated sites are present in the liquid hydrocarbon
For the profiles presented here, lines in the main Q1 branch were used. These lines probe population in one lambda doublet of the lower (F1) spin–orbit manifold, but as we show below the rotational distributions are generally well-described by a temperature, or combination of temperatures
Summary
Chemical processes at gas–liquid interfaces have received increasing attention in recent years. It was proposed that the scattering mechanism could be divided into two limiting processes. In an impulsive scattering (IS) mechanism, the incoming projectile undergoes one, or at most a few, collisions and returns directly into the gas phase with relatively high translational energy. Experimental studies with spectroscopic detection[5,6,7,8] have shown that scattered TD products accommodate their rotational, but not vibrational, energy with the surface. The validity of this empirical binary division between IS and TD has later been questioned in
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