Elucidating the Formation of Styrene via the Gas-Phase Reaction of the Phenyl Radical with 1,3 Butadiene Under Single Collision Conditions

In both terrestrial and extraterrestrial environments, polycyclic aromatic hydrocarbons (PAHs) are widespread, driving extensive interdisciplinary research.^1,2 They play a significant role in the chemical evolution of the interstellar medium (ISM) and are considered a crucial reservoir of interstellar carbon. The presence of PAHs is inferred from the unidentified infrared (UIR) emission bands (3-14 µm) and the diffuse interstellar bands (DIBs), although the identification of individual PAH molecules remains a great challenge.

This broad interest stems from the need to understand the complex pathways leading to PAH formation from their atomic and radical constituents. Central to PAH synthesis is the phenyl radical (C₆H₅) in its ²A₁ ground electronic state, recognized as a key transient species. This has prompted comprehensive investigations into C₆H₅ reactions with unsaturated hydrocarbons from both theoretical and experimental perspectives.^3-5 Recently, our focus shifted to exploring the potential of the reaction between C₆H₅ and 1,3 butadiene as a pathway for producing C₁₀H₁₀ isomers, indene, and styrene.

Using the crossed molecular beam (CMB) scattering technique, combined with mass spectrometric detection and time-of-flight analysis, we meticulously identified primary products and their relative yields (branching fractions, BFs), elucidating the reaction micro-mechanism. Notably, at a collision energy of 141 kJ/mol, we observed the dominance of H-displacement channels, yielding C₁₀H₁₀ + H (BF = 0.77). By comparing our findings with previous CMB experiments and theoretical calculations of the C₁₀H₁₀ potential energy surface, we highlighted the significance of 1-phenyl-cis/trans-1,3-butadiene isomeric products.

Remarkably, our investigation uncovered, for the first time under single collision conditions, the presence of the styrene + vinyl product channel (BF = 0.23). The interest in the styrene molecule arises from its capability to react with another phenyl radical, thereby contributing to the formation of larger PAHs through PAH growth processes. This phenomenon occurs in warm regions of the ISM, such as the circumstellar envelopes of carbon-rich stars like IRC+10216, and proto-planetary nebulae like CRL 618.⁶ 

 

Acknowledgments: We acknowledge financial support under the National Recovery and Resilience Plan (NRRP), Mission 4, Component 2, Investment 1.1, Call for tender No. 104 published on 02.02.2022 by the Italian Ministry of University and Research (MUR), funded by the European Union – NextGenerationEU– Project Title 20227W5CLJ Biomass gasification for hydrogen production (Bio4H2) – CUP J53D23001970006 - Grant Assignment Decree No. 961 adopted on 30.06.2023 by the Italian Ministry of Ministry of University and Research (MUR) and P.C. acknowledges the European Union – NewxtGenerationEU under the Italian Ministry of University and Research (MUR) National Innovation Ecosystem grant ECS0000041 – VITALITY. CUP: B43C22000470005.

 

References

[1] A. M. Nienow and J.T Roberts, Ann. Rev. Phys. Chem., 57 105-128 (2006).

[2] R.I. Kaiser et al., Ann. Rev. Phys. Chem., 66 43-67 (2015).

[3] J.H. Seinfeld and J.F. Pankow, Ann. Rev. Phys. Chem., 54 121-140 (2003).

[4] A.M. Mebel et al., J. Phys. Chem. A, 121 901-926 (2017) (and references therein).

[5] X. Gu et al., J. Phys. Chem. A, 113 998-1006 (2009) (and references therein).

[6] J. Cernicharo et al., Astrophys. J., 546, L123-L126 (2001).