Radboud researchers find possible explanation for abundance of complex organics in cold regions of space
Polycyclic aromatic hydrocarbons (PAHs) are abundant in many regions of the universe, representing a major reservoir for cosmic carbon. However, their formation pathways in cold regions of space remained elusive. Until now. Experimental studies at HFML-FELIX at Radboud University recently provided evidence for alternative formation routes of PAHs in cold environments of the universe like molecular clouds or planetary atmospheres. The results have been published in Nature Astronomy.
Complex molecules in stellar nurseries
Polycyclic aromatic hydrocarbons (PAHs), in principle large molecules made up from fused benzenoid rings, are produced as toxic pollutants on Earth in combustion processes, e.g., in burning coal, oil, gas, garbage and tobacco. In space, they are among the most complex organic molecules detected. Their presence was hypothesized since more than 30 years from the observation of infrared emission in many regions of the universe, attributed to light emitted via vibrational motion of these large aromatic species. Very recently, radio-astronomical observations could unambiguously identify several PAHs, including those containing nitrogen, in a very cold region of space, a molecular cloud in the Taurus constellation 430 lightyears away from Earth. These molecular clouds are the birthplaces of new stars and planets, and the detailed study of their chemistry allows astronomers to better understand the process of star and planet formation. What puzzled the astronomers was that their astrochemical models drastically underestimated the abundance of the detected PAHs and other aromatic species in the cold environment of the molecular cloud. A very likely reason for this is that several important formation routes for PAHs are missing in these networks, because they were not studied previously either by theory or experimentally.
The researchers at HFML-FELIX set out to study a specific class of promising reactions, those between a charged molecule (cation) and a neutral molecule. They chose the pyridine cation, basically a monocyclic benzene ion where one C-H unit is replaced by a nitrogen atom, as a starting point, and investigated its reaction with neutral acetylene (C2H2). For this, a cold ion-trap instrument stationed at HFML-FELIX was used which mimics the conditions of molecular clouds, i.e., cold temperatures and low densities. The team found through mass-spectrometric kinetic studies that molecules with larger mass are indeed efficiently formed in this reaction. Up to two acetylene units could be attached to the original pyridine cation. However, by only detecting the mass of a certain molecule, one does not obtain any information of its structure. Was the product indeed a PAH, or just a pyridine with some dangling acetylene units?
The research team succeeded to answer this question by combining low-temperature kinetic studies and in-situ infrared spectroscopic probing using an infrared free-electron laser at HFML-FELIX with quantum-chemical calculations. Only the combination of kinetic and spectroscopic results of this study provides the unambiguous experimental proof for the formation of the polycyclic quinolizinium ion from a monocyclic precursor. Furthermore, the spectroscopic identification of reaction intermediates allowed the researchers to disentangle competing formation pathways, providing information beyond purely mass-spectrometric and computational studies. The observed quinolizinium ion belongs to an astronomically interesting class of nitrogen containing PAHs that are proposed to contribute to the observed infrared emission seen in many regions of space. The obtained spectroscopic data can thus act as a basis for future astronomical observations aimed to unravel the formation of PAHs in space, e.g., with the recently launched James Webb Space Telescope.
Sandra Brünken: "We believe this work conveys an important scientific result towards our understanding of astrochemical processes not only in the local interstellar medium but also in extragalactic and (exo-) planetary environments. Furthermore, it presents a general and novel experimental method with a broad applicability in the field to study, e.g., formation routes towards prebiotic molecules like sugars or amino acids, or processes relevant in plasma and combustion science."
Figure: Schematic reaction scheme from monocyclic pyridine radical cation to the polycyclic quinolizinium (left), as elucidated by FELIX infrared spectroscopy of the reaction intermediates and the final product (right).
At HFML-FELIX, high magnetic fields and intense (far) infrared free electron lasers are designed and used to investigate the properties and functionality of molecules and materials, realize fundamental scientific breakthroughs and tackle societal challenges in the areas of health, energy and smart materials. HFML-FELIX is scientifically embedded in the Institute for Molecules and Materials (IMM) within the Faculty of Science at Radboud University. (Bio)chemists, physicists, theorists and experimentalists, work closely together to unravel and control the functioning of molecules and materials at the smallest length and time scales.
PhD student Daniël Rap, first author of the study, at FELIX (copyright: HFML-FELIX)
Daniël B. Rap, Johanna G.M. Schrauwen, Aravindh N. Marimuthu, Britta Redlich, Sandra Brünken
Nature Astronomy (2022)