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Abstract
The reactions of ground state atomic oxygen, O(3P), with unsaturated hydrocarbons (UHs) are of relevance in the oxidation in different environments. They are usually multichannel reactions that exhibit a variety of competing product channels, some of which occur adiabatically on the entrance triplet potential energy surface (PES), while others occur nonadiabatically on the singlet PES that can be accessed via intersystem crossing (ISC). ISC plays a key role on the mechanism of these reactions, impacting greatly the product yields. Understanding the mechanism of O(3P) reactions with UHs requires the identification of all primary reaction products, the determination of their branching fractions (BFs), and the assessment of the role of ISC. This goal can be best achieved combining crossed-molecular-beam (CMB) experiments with universal, soft ionization, mass-spectrometric detection and time-of-flight analysis to high-level ab initio electronic structure calculations of triplet/singlet PESs and Rice-Ramsperger-Kassel-Marcus/Master Equation (RRKM/ME) computations of product BFs with inclusion of ISC effects. In our laboratory this kind of approach was found to be successful for O(3P) reactions with the simplest UHs (alkynes, alkenes, dienes) containing two, three, or four carbon atoms. Here, we report the full experimental/theoretical work on the O(3P) + 1,2-butadiene reaction that allows us to explore how the mechanism and product distribution vary when moving from O(3P) + propadiene (allene) to O(3P) + 1,2-butadiene (methylallene) and when we compare this system to related C4 unsaturated systems, namely O(3P) + 1-butene and O(3P) + 1,3-butadiene. In the present work, a total of nine product channels were observed and characterized in a CMB experiment at the collision energy of 41.8 kJ/mol. Synergistic ab initio transition-state theory-based master equation simulations coupled with nonadiabatic transition-state theory on coupled triplet/singlet PESs were employed to compute product BFs and assist the interpretation of the CMB experimental results. Good agreement is found between theoretical predictions and experimental results. The finding of this work can be useful for kinetic modeling of 1,2-butadiene oxidation and of systems where 1,2-butadiene is an important intermediate.
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Supporting Information for the paper
Description
Figure S1. Triplet PES for the O(³P) + CH₂=C=CH–CH₃ reaction (C1 attack), including additional pathways vs. Figure 14.
Figure S2. Singlet PES for the title reaction (C1 attack), including additional pathways vs. Figure 14.
Figure S3. Triplet PES for the title reaction (C2 attack), including additional pathways vs. Figure 15.
Figure S4. Singlet PES for the title reaction (C2 attack), including additional pathways vs. Figure 15.
Figure S5. Triplet PES for the title reaction (C3 attack), including additional pathways vs. Figure 16.
Figure S6. Singlet PES for the title reaction (C3 attack), including additional pathways vs. Figure 16.
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