Investigating H-atom reactions in small PAHs with imperfect aromaticity: A combined experimental and computational study of indene (C₉H₈) and indane (C₉H₁₀).

This article focuses on the experimental and theoretical investigation of the reactivity of indene (C₉H₈) and its hydrogenated form, indane (C₉H₁₀), with hydrogen atoms at low temperatures, assessing their potential catalytic role in the formation of molecular hydrogen (H₂) in the interstellar medium (ISM). Using the para-H₂ matrix isolation technique at 3.1 K, the study demonstrates that indene undergoes both H-atom-abstraction and H-atom-addition reactions, producing the 1-indenyl radical (R1) and the 1,2-dihydro-indene-3-yl radical (R2), respectively, while indane primarily participates in H-atom-abstraction reactions forming the R2 radical. The research reports, for the first time, the infrared spectra of these radicals and suggests a quasi-equilibrium catalytic cycle involving indene, indane, and their radicals that may facilitate interstellar H₂ formation. These findings contribute to understanding the chemical pathways of H₂ synthesis in cold astrophysical environments and support the role of imperfect aromatic molecules as effective catalysts in the ISM.