Polyaromatic hydrocarbons with an imperfect aromatic system as catalysts of interstellar H₂ formation

Abstract

Although H${}_2$ is the simplest and the most abundant molecule in the Universe, its formation in the interstellar medium, especially in the photodissociation regions is far from being fully understood. According to suggestions, the formation of H${}_2$ is catalysed by polyaromatic hydrocarbons (PAHs) on the surface of interstellar grains. In the present study we have investigated the catalytic effect of small PAHs with an imperfect aromatic system. Quantum chemical computations were performed for the H-atom-abstraction and H-atom-addition reactions of benzene, cyclopentadiene, cycloheptatriene, indene, and 1H-phenalene. Heights of reaction barriers and tunnelling reaction rate constants were computed with density functional theory using the MPWB1K functional. For each molecule, the reaction path and the rate constants were determined at 50 K using ring-polymer instanton theory, and the temperature dependence of the rate constants was investigated for cyclopentadiene and cycloheptatriene. The computational results reveal that defects in the aromatic system compared to benzene can increase the rate of the catalytic H${}_2$ formation at 50 K.

GRAPHICAL ABSTRACT

Keywords:

• H atom tunnelling
• instanton theory
• astrochemistry
• polyaromatic hydrocarbons
• interstellar H${}_2$ formation

Acknowledgments

This paper is dedicated to Professor Péter G. Szalay on the occasion of his 60th birthday. The support of the Lendület program of the Hungarian Academy of Sciences is acknowledged.

Data availability statement

Supplemental Material: Tables S1–S11: The rate constants, the number of the beads, and the crossover temperatures corresponding the instanton geometries of species discussed in the paper. Table S12: MPWB1K/cc-pVTZ structures (Cartesian coordinates in Å), energies and computed harmonic vibrational frequencies (in cm${}^{-1}$) and IR intensities (km mol${}^{-1}$) of species discussed in the paper. The research data supporting this publication can be accessed at https://doi.org/10.1080/00268976.2022.2142168

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by the MTA Lendület and the ELTE Institutional Excellence Programs. The authors acknowledge Dr. Yair Litman for helpful discussions about the ring-polymer instanton technique. The authors appreciate the computational resources provided by the ELTE IIG High-Performance Computing facility.