Superhydrogenation of indene at low temperatures

¹ Institute for Theoretical Chemistry, University of Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, Germany
² Current Address: Department of Engineering, University of Perugia, Via Duranti 93, 06125 Perugia, Italy

^★ Corresponding author: campisi@theochem.uni-stuttgart.de

Received: 19 July 2024
Accepted: 19 June 2025

Abstract

Context. The hydrogenation of polycyclic aromatic hydrocarbons (PAHs) is crucial to understanding molecular hydrogenation formation in the interstellar medium (ISM). This process also helps to elucidate the weakening of the aromatic bonds in PAHs, which may function as a carbon reservoir, facilitating the formation of interstellar complex organic molecules (iCOMs) through top-down chemistry. Tunneling can significantly promote the hydrogenation process in a low-to-moderate temperature range (approximately 10–200 K), which could also be important in warmer regions of the ISM, such as photodissociation regions (PDRs).

Aims. We aim to present the hydrogenation sequence of the newly observed PAH molecule, indene, for the first time and clarify the tunneling rule at temperature in PDR and dark molecular-cloud conditions. In addition, we report fit parameters to be utilized in astronomical modeling.

Methods. The hydrogenation sequence was studied using simple hydrogenation rules based on tight-binding methods and confirmed by barriers from density functional theory (DFT). The binding energy, activation energies, kinetic rate constants, and tunneling corrections–based on the Bell and Eckart models and supported by the accurate instanton method–were calculated using DFT. To make our kinetic studies useful to modelers, we implemented a Monte Carlo method-based program to generate and optimize random initial fit parameters (α, β, γ, and T₀) to achieve the statistically best fit.

Results. We find that indene hydrogenation proceeds with saturation of carbon atoms in the pentagonal ring first, followed by hydrogenation of the benzene unit. Indene hydrogenation follows rules similar to those of other PAHs, such as pentacene, coronene, and corannulene, with binding energies for odd-numbered hydrogenation steps ranging from 0.5 to 2 eV and barriers around 0.13 eV for the first, fifth, and seventh hydrogenation steps. The third hydrogenation step is the rate-limiting step, with a barrier of 0.24 eV, similar to what is found for other PAHs. Even-numbered hydrogenation steps have lower barriers and lead to more stable intermediates as a result of radical-radical recombination. The hydrogenation sequence follows a scheme that strongly depends on the PAH’s shape, the number of aromatic rings, and the presence of five-membered rings, aiming to preserve the aromaticity as much as possible. Furthermore, we observe that tunneling plays an important role in the hydrogenation of indene at temperatures between 30 and 75 K, which corresponds to the temperatures of dust in PDRs. Finally, our implementation includes fit parameters that reproduce our model with a high degree of accuracy, achieving a static precision of 0.99(R²) and an RMS error of 10⁻².