While polycyclic aromatic hydrocarbons (PAHs) are now accepted to be abundant in interstellar space, the abundance and influence of superhydrogenated PAHs (HPAHs) in the interstellar medium (ISM) are still under investigation. HPAHs may act as catalysts for or reactants in small-molecule formation via hydrogen abstraction reactions, H₂ evaporation, and carbon skeleton fragmentation. Here, we present a gas-phase infrared (IR) action spectroscopy study of the HPAH 4, 5, 9, 10-tetrahydropyrene (THP; C₁₆H₁₄), performed at the Free Electron Lasers for Infrared eXperiments facility. IR action spectroscopy was performed on the THP cation, protonated THP, and their fragments produced by collision-induced dissociation in the range from 600 to 1800 cm⁻¹. Calculated IR spectra, at the density functional theory level, agree with experimental IR spectra to a high degree and were utilized to determine molecular structures of the HPAH fragments. Molecular dynamics simulations compared with experimental mass spectra reveal favorable HPAH fragmentation pathways. Molecular hydrogen (H₂) is observed to be a primary fragment of [THP+H]⁺ with superhydrogenated duo groups. This contrasts the notion that HPAHs typically undergo carbon skeleton fragmentation leading to C _x H _y formation. These observations show that lowered symmetry and duo or trio aliphatic groups on HPAHs uniquely change their IR spectra, stability, and fragmentation patterns. As a result, these species may contribute to H₂ formation in the ISM.

中文翻译：

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通过部分超氢化芘振动激发形成分子氢


虽然多环芳烃 (PAH) 现在被认为在星际空间中含量丰富，但超氢化 PAH (HPAH) 在星际介质 (ISM) 中的丰度和影响仍在研究中。 HPAHs可以作为通过夺氢反应、H ₂蒸发和碳骨架断裂形成小分子的催化剂或反应物。在这里，我们介绍了在自由电子激光器红外实验设施中进行的 HPAH 4,5,9,10-四氢芘（THP；C ₁₆ H ₁₄ ）的气相红外（IR）作用光谱研究。对THP阳离子、质子化THP以及它们在600至1800cm ^-1范围内由碰撞诱导解离产生的片段进行红外作用光谱。计算出的红外光谱在密度泛函理论水平上与实验红外光谱高度一致，可用于确定 HPAH 片段的分子结构。分子动力学模拟与实验质谱相比揭示了有利的 HPAH 碎片途径。观察到分子氢 (H ₂ ) 是具有超氢化双基团的 [THP+H] ⁺的主要片段。这与 HPAH 通常经历碳骨架断裂导致 C _x H _y形成的观点形成鲜明对比。这些观察结果表明，HPAH 上的对称性降低以及二重或三重脂肪族基团会独特地改变其红外光谱、稳定性和碎片模式。结果，这些物质可能有助于ISM中H _{2 的}形成。