Gas-phase chemistry of C 3 H 3+: a key building block for carbon mass growth in space

The gas-phase chemistry of C3H+3 plays a central role in carbon mass growth across various environments, and this work combines tunable vacuum-ultraviolet (VUV) photoionization with controlled ion–molecule reaction measurements to selectively probe the reactivity of its isomers.

The gist: The most stable cyclic isomer is found to be essentially unreactive under the investigated conditions, whereas the linear isomer exhibits efficient and diverse chemistry leading to the formation of larger hydrocarbon ions.

Isomer-Specific Probing Method

The study addresses the limitation that multiple isomers, primarily the linear propargyl cation and the cyclic cyclopropenyl cation, have prevented reliable determination of isomer-specific reaction kinetics By employing a titration approach, researchers extracted isomer-specific reaction rate coefficients and branching ratios for reactions of C3H+3 with C2H4, C3H4, and C3H6. This method relies on the ability to successively produce cC3H+3 alone and then a mixture of cyclic and linear forms using vacuum-ultraviolet (VUV) dissociative photoionization. This approach allows for the determination of new isomer-specific reaction rate coefficients and refines previously reported values that were convoluted by isomeric mixtures.

Reactivity Contrast Between Isomers

The results demonstrate a "strong contrast in reactivity: the most stable cyclic isomer is found to be essentially unreactive under the investigated conditions, whereas the linear isomer exhibits efficient and diverse chemistry leading to the formation of larger hydrocarbon ions. The linear isomer is shown to exhibit efficient and diverse chemistry leading to the formation of larger hydrocarbon ions, specifically observing the formation of C6H+5 is observed, providing an important intermediate in pathways toward aromatic species".

Reactions with Hydrocarbons

The study investigated the reactivity of C3H+3 with three different hydrocarbons:

1. With C2H4, the reaction cross sections and rate coefficients were presented as a function of photon energy, showing that the propargyl structure is significantly more reactive than the cyclic form. The reaction channels correspond to the formation of C3H+5 and C5H+5 (loss of C2H2 or H2 from the adduct intermediate), with an approximately equal branching ratio at the AE threshold, which can be correlated to a temperature of 300 K.

2. With C3H4 (allene and propyne), four reaction channels were identified, ordered by decreasing branching ratio at the propargylium AE: C6H+5, C4H+5, C6H+6, and C4H+3. The formation of C6H+5 from propyne provides the missing link in the proposed benzene-formation pathway in dense molecular clouds of Loison et al. (2026a).

3. With C3H6, six reaction channels were identified, and all the branching ratios except C2H+5 remain essentially constant, as expected if all channels proceed through the same adduct formation and share a common rate-limiting step.

Astrochemical Implications

The isomer-specific rate coefficients and branching ratios provide more robust constraints for kinetic and astrochemical models, particularly in environments where ion–molecule reactions govern molecular complexity. Among the three studied reactions, the l-C3H+3 + C3H4 reaction is the most significant, as it initiates a reaction sequence (followed by C6H+5 + H2 and then C6H+7 + e–) that constitutes the dominant pathway towards benzene formation in Nautilus. The model reproduces well the observed abundance of C6H5CN and slightly overestimates that of C6H5C2H. The reaction (c,l−)C3H+ + H2 plays a crucial role in the formation of the smallest C6 aromatic compounds. This study emphasizes the critical importance of accounting for isomer-specific chemistry in the description of carbon growth processes and demonstrates the capability of VUV-based approaches to probe complex ion–molecule reaction networks with high selectivity and quantitative accuracy".

Conclusion

The work confirms that while c-C3H+3 is essentially unreactive, l-C3H+3 exhibits a rich and efficient chemistry with unsaturated hydrocarbons, highlighting its role as a key intermediate in carbon mass growth and in the formation of aromatic precursors. The isomer-specific rate coefficients and branching ratios reported offer more robust constraints for kinetic and astrochemical models, particularly in environments where ion–molecule reactions govern molecular complexity. Overall, this study underscores the critical importance of accounting for isomer-specific chemistry in the description of carbon growth processes and demonstrates the capability of VUV-based approaches to probe complex ion–molecule reaction networks with high selectivity and quantitative accuracy".

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Gas-phase chemistry of C3H+3: a key building block for carbon mass growth in space Corentin Rossia, Jean-Christophe Loisonb, Anne P. Rasmussena,c, Roland Thissena,d,e, Nandana Pattathadathild,f, Christian Alcarazd,e Ugo Jacovellaa aUniversité Paris-Saclay; CNRS Institut des Sciences Moléculaires d’Orsay; Orsay 91405 France bUniversité Bordeaux CNRS Bordeaux INP ISM UMR 5255 Talence F-33400 France cDepartment of Physics and Astronomy Aarhus University Aarhus C 8000 Denmark dUniversité Paris-Saclay CNRS Institut de Chimie Physique UMR8000 Orsay 91405 France eSOLEIL L’Orme des Merisiers Saint-Aubin 91192 France f Department of Physics University of Trento Via Sommarive 14 Trento 38123 Italy Abstract The gas-phase chemistry of C3H+3 plays a central role in the growth of carbonaceous species across a wide range of environments, from combustion systems to interstellar media. However, the presence of multiple isomers, primarily the linear propargyl cation and the cyclic cyclopropenyl cation, has so far prevented the determination of reliable isomer-specific reaction kinetics. In this work, we combine tunable vacuum-ultraviolet (VUV) photoionization with controlled ion–molecule reaction measurements to selectively produce and probe the reactivity of C3H+3 isomers. By employing a titration approach, we extract isomer-specific reaction rate coefficients and branching ratios for reactions of C3H+3 with C2H4, C3H4, and C3H6. Our results demonstrate a strong contrast in reactivity: the most stable cyclic isomer is found to be essentially unreactive under the investigated conditions, whereas the linear isomer exhibits efficient and diverse chemistry leading to the formation of larger hydrocarbon ions. In particular, the formation of C6H+5 is observed, providing an important intermediate in pathways toward aromatic species. Email address: ugo.jacovella@cnrs.fr (Ugo Jacovella) arXiv:2610.11985v1 [physics.chem-ph] 8 Oct 2026

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These findings help resolve long-standing ambiguities in previously reported rate coefficients and provide new quantitative constraints for kinetic and astrochemical models. More broadly, this work highlights the importance of isomer-specific chemistry in carbon growth processes and demonstrates the effectiveness of VUV-based photoionization methods for disentangling complex reaction networks in the gas phase. Keywords: Mass spectrometry, Ion-molecule reactions, Astrochemical modeling, Synchrotron radiation

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1. Introduction Understanding the molecular processes that govern early-stage carbon mass growth remains a central challenge in carbon science spanning environments as diverse as combustion systems plasma reactors and astrophysical media. Among the key intermediates implicated in these processes, the C3H+3 hydrocarbon ion occupies a particularly important position. It acts as an efficient conduit for the conversion of small hydrocarbons into larger carbonaceous species (Smyth et al., 1982; Herbst et al., 1983; Baykut et al., 1986; Loison et al., 2017; Cherepanov et al., 2024). Beyond combustion, C3H+3 is well established as a key intermediate in astrochemical reaction networks that drive molecular complexity in low-temperature environments (Loison et al.

1. Identify isomer-specific reaction kinetics through tunable VUV photoionization combined with controlled ion–molecule reaction measurements to selectively produce and probe the reactivity of C3H+3 isomers. This allows AI systems to move beyond convoluted averages by determining isomer-specific reaction rate coefficients and branching ratios for reactions of C3H+3 with C2H4, C3H4, and C3H6.

2. Develop astrochemical models capable of incorporating isomer-specific data from the paper to resolve ambiguities in previously reported kinetic models. This improved system can provide new quantitative constraints for kinetic and astrochemical models by using the isomer-specific rate coefficients and branching ratios reported here.

3. Improve reaction mechanism simulations by incorporating insights derived from the experimental results regarding transition states and energy barriers. The AI can utilize data showing that the formation of C6H+5 from propyne provides the missing link in the proposed benzene-formation pathway in dense molecular clouds to refine models of carbon growth.

4. Enhance predictive capabilities for interstellar chemistry by testing how different reaction pathways affect aromatic abundances under various conditions. The system can simulate scenarios where removing this reaction [l-C3H+ + C3H4] leads to a substantial decrease in aromatic abundances, by approximately a factor of 2.5.

5. Create a diagnostic tool for identifying the dominant formation routes of specific carbonaceous species based on internal energy deposition profiles. The AI can analyze Figure 6: Branching ratios as a function of the maximum internal energy for the reaction of l-C3H+ with propyne and allene to determine which channel, such as The channel leading to C6H+5 is favored at low internal energies, which are the most representative conditions in dense molecular clouds such as TMC-1.