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

Although H2 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 H2 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 H2 formation at 50 K.