How Cosmic Carbon Rings are Born: Ion Chemistry in Cold Clouds ✦
Using the multipurpose PIRENEA setup, the reactivity of benzylium-type isomers C₁₁H₉⁺ with dimethylnaphthalene was investigated under isolated conditions mimicking radiative association. Products C₁₂H₁₁⁺ and C₁₃H₁₃⁺ were detected, their ratio depending on the specific dimethylnaphthalene isomer. Subsequent UV-visible irradiation allowed the identification of the acenaphthylene radical cation C₁₂H₈•⁺ as the most stable photofragment. Density functional theory calculations and molecular dynamics simulations confirm that long-lived ion-molecular complexes promote the formation of C-C bonds and the closure of a five-membered ring. The results point to a new pathway for the formation of five-membered rings during the growth of polycyclic aromatic hydrocarbons under low-pressure and low-temperature conditions, which may explain the recent detection of acenaphthylene-like particles in the cold cloud TMC-1.
Context
Polycyclic aromatic hydrocarbons (PAHs) are among the most common organic molecules in the universe. They make up a significant portion of cosmic dust, influence the thermal balance of clouds, and serve as raw material for prebiotic chemistry. However, the pathways of their formation in cold conditions (about 10 K) remained a mystery. Recent discoveries with radio telescopes and the James Webb Space Telescope showed that even in the darkest corners of space, complex PAHs exist, including cyano-substituted forms. This requires a revision of chemical evolution models.
Methods
The researchers used a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) as part of the PIRENEA setup. C₁₁H₉⁺ ions were produced by photodissociation of dimethylnaphthalenes and isolated in high vacuum. They were then mixed with neutral molecules at room temperature, simulating radiative association processes — the main mechanism for molecular growth in rarefied environments. The reaction products were irradiated with visible light, and their structure was investigated using time-of-flight mass spectrometry and multiphoton dissociation spectroscopy. Quantum chemical calculations by density functional theory and molecular dynamics (DFTB) allowed modeling the dynamics of ion-molecule complexes and confirming the formation of five-membered rings.
Results
Main result: C₁₁H₉⁺ ions (benzylic isomers of naphthalene) react with dimethylnaphthalene, yielding two products: C₁₂H₁₁⁺ and C₁₃H₁₃⁺. Their ratio depends on the position of methyl groups. For example, for 2,3-dimethylnaphthalene the fraction of C₁₃H₁₃⁺ was 79%, while for 1,4- and 1,5-isomers C₁₂H₁₁⁺ dominates (70% and 83%). Upon subsequent irradiation with visible light, both products lose hydrogen or methyl groups, ultimately forming the acenaphthylene cation (C₁₂H₈⁺) — an exceptionally photostable species. This is confirmed by comparison with literature data: the acenaphthylene cation withstands heating up to an energy of 7.6 eV, which is comparable to anthracene and much higher than that of naphthalene (5.8 eV).
Implications
The discovery points to a new pathway for the formation of five-membered carbon rings in the interstellar medium. Previously it was thought that such structures only arise at high temperatures, but these experiments demonstrate an efficient low-temperature mechanism via long-lived ion-molecule complexes. This directly explains the presence of cyanoacenaphthylene in the cold cloud TMC-1 — previously no synthesis pathway was known for it.
Future development
In the future, similar experiments are planned at cryogenic temperatures to closely approach the conditions of molecular clouds. It is also important to investigate reactions with other alkyl-PAHs and radicals like CN to build a complete picture of aromatic system growth. Joint use of data from James Webb and ground-based radio telescopes will allow testing the predicted PAH formation pathways in various astrophysical environments — from cold cores to photodissociation regions.
Impact
The results will impact astrochemistry, interstellar medium astrophysics, and planetary science — especially in understanding the chemistry of Titan's atmosphere, where similar ions were detected by the Cassini probe.
Next steps
The next step will be experimental study of C₁₁H₉⁺ reactions with other neutral partners, including benzene and naphthalene, as well as direct detection of intermediate complexes using infrared spectroscopy in traps.
Key open problems
The work connects two fundamental problems: the growth of complex organic molecules at low temperatures and the origin of unidentified infrared bands (UIR), attributed to PAHs. Understanding the formation pathways of pentacycles is key to unraveling the composition of cosmic dust.
🎯 Interestingly, acenaphthene is a component of coal tar, which in the 19th century was used for dye production. Now its 'cosmic relatives' help us understand the chemistry of star birth.
🎬 The role of PAHs as building blocks of prebiotic chemistry recalls Fred Hoyle's novel 'The Black Cloud', where an interstellar gas-dust cloud turns out to be a living organism. Today's science shows that clouds are indeed full of complex organics.
Key numbers
- fraction of C₁₃H₁₃⁺ from 2,3-dimethylnaphthalene: 79%
- fraction of C₁₂H₁₁⁺ from 1,4-dimethylnaphthalene: 70%
- photostability energy of acenaphthylene cation: 7.6 eV
- temperature of cold molecular clouds: ~10 K