光解诱导的多环芳烃乱序作为氘储存机制
Photolysis-induced scrambling of PAHs as a mechanism for deuterium storage
摘要：
瞄准我们调查的作用多环芳烃作为一个汇的氘在星际介质和研究紫外光解作为一个潜在的过程中的变化的氘分馏在ISM。方法.在FTICR质谱仪上记录了各种富含D的质子化蒽和菲离子的同位素体的紫外光诱导裂解。使用FELIX的IRMPD光谱提供了与DFT振动光谱相比较的IR光谱;还计算了反应势垒和速率，并与产物丰度相关。结果UV和IRMPD光解的质谱均显示H从[D-C $_{14}$ H $_{10}$ ] $^+$中损失，而[H-C $_{14}$ D $_{10}$ ] $^+$显示出对D损失的强烈偏好。计算揭示了容易的1，2-H和-D移位反应，与障碍低于由光激发过程提供的能量。通过红外光谱的基态结构的确认一起，我们确定，在2多环芳烃的光解过程在很大程度上是由混乱的H和D原子之间的不同的外围C原子重新定位。C-H键和C-D键之间零点能的$\sim$ 0.1 eV差异最终导致H扰乱比D扰乱更快，并且与D相比H原子损失增加。结论PAH阳离子在紫外线照射下通常会发生无序排列。富氘多环芳烃的光激发后，混乱的结果在一个更高的概率为脂肪族D原子迁移到一个芳香族的网站，保护它从消除。这可能会导致更多的氘作为PAH移动到更暴露的星际环境。此外，在solo站点上具有脂肪族C-HD基团的大型紧凑的多环芳烃可能是天文光谱中看到的大多数脂肪族C-D伸缩带的原因。
英文摘要：
Aim. We investigate the role of PAHs as a sink for deuterium in the interstellar medium and study UV photolysis as a potential process in the variations of the deuterium fractionation in the ISM. Methods. The UV photo-induced fragmentation of various isotopologues of D-enriched, protonated anthracene and phenanthrene ions was recorded in a FTICR mass spectrometer. IRMPD spectroscopy using FELIX provided the IR spectra that were compared to DFT vibrational spectra; reaction barriers and rates were also calculated and related to the product abundances. Results. The mass spectra for both UV and IRMPD photolysis show the loss of H from [D-C$_{14}$H$_{10}$]$^+$, whereas [H-C$_{14}$D$_{10}$]$^+$ shows a strong preference for D loss. Calculations reveal facile 1,2-H and -D shift reactions, with barriers lower than the energy supplied by the photo-excitation process. Together with confirmation of the ground-state structures via the IR spectra, we determined that the photolytic processes in the 2 PAHs are largely governed by scrambling where the H and the D atoms relocate between different peripheral C atoms. The $\sim$0.1 eV difference in zero-point energy between C-H and C-D bonds ultimately leads to faster H scrambling than D scrambling, and increased H atom loss compared to D. Conclusion. Scrambling is common in PAH cations under UV radiation. Upon photoexcitation of deuterium-enriched PAHs, the scrambling results in a higher probability for the aliphatic D atom to migrate to an aromatic site, protecting it from elimination. This could lead to increased deuteration as a PAH moves towards more exposed interstellar environments. Also, large, compact PAHs with an aliphatic C-HD group on solo sites might be responsible for the majority of aliphatic C-D stretching bands seen in astronomical spectra.
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