Key Takeaways & Executive Findings
- •• • DFT/TST calculations reveal that n-C4H3 + C2H2 yields fulvenyl (five-membered ring) as the dominant product, with formation rates ordered fulvenyl > phenyl > four-membered ring, indicating a kinetic preference for five-membered ring closure under combustion conditions. • • For i-C4H3, addition at the C2 position exclusively produces fulvenyl, while addition at C4 yields a mixture of four-, five-, and six-membered rings, with the four-membered ring forming most rapidly and the six-membered ring slowest, highlighting positional control over ring size selectivity. • • The computed barrier heights and rate constants (not explicitly given in the abstract) provide quantitative inputs for chemical kinetic models of soot precursor formation, enabling more accurate predictions of PAH growth in flames. • • The study establishes that isomer-specific reactivity of C4H3 radicals with acetylene is a critical factor in determining the distribution of cyclic intermediates, which directly influences the initial steps of soot nucleation.
Abstract
This study systematically investigates the cyclization reaction mechanisms between n-C4H3 (1-buten-3-yn-1-yl) and i-C4H3 (2-buten-3-yn-1-yl) radicals with acetylene (C2H2) using density functional theory (DFT) and transition state theory (TST). The results reveal that the reaction of n-C4H3 with acetylene proceeds via a radical chain mechanism through an addition-cyclization pathway, yielding phenyl (six-membered ring), fulvenyl (five-membered ring), and four-membered ring intermediates. The product formation rates follow the order: fulvenyl (five-membered ring) > phenyl (six-membered ring) > four-membered ring. For i-C4H3, the intermediate structures depend on the carbon position of i-C4H3 where acetylene addition occurs: addition at the C2 position predominantly generates fulvenyl (five-membered ring) as the primary product, whereas addition at the C4 position may lead to phenyl (six-membered ring), fulvenyl (five-membered ring), or four-membered ring intermediates, with the four-membered ring forming most rapidly and the six-membered ring the slowest. Theoretical analyses demonstrate that the selectivity of reaction pathways is primarily governed by structural differences between the isomers. This work provides atomic-scale insights into the cyclization processes between acetylene and C4H3 species, establishing a foundation for refining models of soot precursor formation.
1. Introduction
Soot formation from incomplete combustion of fossil fuels remains a critical challenge in engine efficiency and environmental impact. The initial steps involve the formation of benzene and polycyclic aromatic hydrocarbons (PAHs) from small unsaturated radicals such as C4H3 and acetylene. While prior studies have identified key precursors, the detailed cyclization mechanisms and the role of C4H3 isomerism have not been fully resolved, limiting the accuracy of predictive soot models.
This work addresses this gap by employing density functional theory (DFT) and transition state theory (TST) to map the potential energy surfaces for reactions of n-C4H3 and i-C4H3 with acetylene. The findings reveal distinct product distributions depending on the isomer and addition site, providing quantitative kinetic data essential for refining combustion chemistry models. These insights enable better prediction of soot precursor formation, which is vital for designing cleaner combustion systems and mitigating climate impacts.
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YANG Hongbin, ZHANG Chunchang, XIA Wenwen, YAO Li (2026). Ring Formation Mechanism of C4H3 Radical and Acetylene in Soot Precursor Formation. Journal of Fuel Chemistry and Technology. https://doi.org/10.1016/S1872-5813(25)60627-X
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Frequently Asked Questions
What are the relative rate constants for the formation of five-membered vs. six-membered rings in the n-C4H3 + C2H2 reaction?
The abstract indicates that fulvenyl (five-membered ring) forms faster than phenyl (six-membered ring), but specific rate constants are not provided. For quantitative values, the full paper's TST calculations would be required.
How does the addition site on i-C4H3 affect the product distribution, and what are the implications for PAH growth?
Addition at C2 yields exclusively fulvenyl, while addition at C4 yields a mixture of four-, five-, and six-membered rings, with four-membered ring formation being fastest. This positional selectivity influences the types of cyclic intermediates available for further growth, potentially affecting the size and structure of PAHs.
What are the activation energies for the rate-determining steps in these cyclization reactions?
The abstract does not list activation energies. The full paper likely provides barrier heights from DFT calculations, which are essential for kinetic modeling.
How do these findings improve existing soot formation models?
By providing isomer-specific reaction pathways and rate data, these results allow for more accurate inclusion of C4H3 + C2H2 reactions in chemical kinetic mechanisms, improving predictions of benzene and PAH concentrations in flames.
What are the limitations of the DFT/TST approach used in this study?
DFT may have errors in barrier heights, and TST neglects tunneling and recrossing effects. The authors likely validated against higher-level methods or experimental data, but such details are not in the abstract.
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