Lopes Cavalcante Larissa, Czaplinski Ellen C, Maynard-Casely Helen E, Cable Morgan L, Chaouche-Mechidal Naila, Hodyss Robert, Ennis Courtney
Department of Chemistry, University of Otago, Dunedin 9054, New Zealand.
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, USA.
Phys Chem Chem Phys. 2024 Oct 30;26(42):26842-26856. doi: 10.1039/d4cp03437f.
Nitrogen-containing polycyclic aromatic hydrocarbons (NPAHs) are important molecules for astrochemistry and prebiotic chemistry, as their occurrence spans from interstellar molecular clouds to planetary systems. Their formation has been previously explored in gas phase experiments, but the role of solid-state chemical reactions in their formation under cryogenic conditions remains elusive. Here, we explore the formation of NPAHs through vacuum ultraviolet (VUV) irradiation of pyridine:acetylene ices in amorphous and co-crystalline phases, with the aim to simulate conditions relevant to the interstellar medium and Titan's atmosphere. Our results show that the synthesis of ethynylpyridines from VUV-irradiated pyridine:acetylene amorphous ices is achievable at 18 K. In the co-crystal phase, photolysis at 110 K leads to the formation of NPAHs such as quinolizinium+ and precursors, reflecting a dynamical system under our conditions. In contrast, irradiation at 90 K under stable conditions did not produce volatile photoproducts. These results suggest that such chemical processes can occur in Titan's atmosphere and potentially in its stratosphere, where the co-condensation of these molecules can form composite ices. Concurrently, the formation of stable co-crystals can influence the depletion rates of pyridine, which suggests that these structures can be preserved and potentially delivered to Titan's surface. Our findings provide insights into the molecular diversity and chemical evolution of organic matter on Titan, crucial for future space exploration missions, such as the Dragonfly mission, which may uncover higher-order organics derived from pyridine precursors on Titan's surface.
含氮多环芳烃（NPAHs）是天体化学和生命起源前化学中的重要分子，因为它们存在于从星际分子云到行星系统的各个区域。此前已在气相实验中探索过它们的形成过程，但低温条件下固态化学反应在其形成过程中的作用仍不清楚。在这里，我们通过对非晶相和共晶相中的吡啶：乙炔冰进行真空紫外（VUV）辐照来探索NPAHs的形成，目的是模拟与星际介质和土卫六大气相关的条件。我们的结果表明，在18K时，通过VUV辐照吡啶：乙炔非晶冰可实现乙炔基吡啶的合成。在共晶相中，110K下的光解导致喹嗪鎓离子等NPAHs及其前体的形成，这反映了我们实验条件下的一个动态系统。相比之下，在稳定条件下90K的辐照未产生挥发性光产物。这些结果表明，此类化学过程可能发生在土卫六的大气中，甚至可能发生在其平流层中，在那里这些分子的共凝结可形成复合冰。同时，稳定共晶的形成会影响吡啶的消耗速率，这表明这些结构可以被保存下来，并有可能被输送到土卫六的表面。我们的发现为土卫六上有机物质的分子多样性和化学演化提供了见解，这对于未来的太空探索任务至关重要，比如蜻蜓任务，该任务可能会在土卫六表面发现源自吡啶前体的高阶有机物。
