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Discovery of Two Cyano Derivatives of Acenaphthylene (C12H8) in TMC-1 Revealed by the QUIJOTE Line Survey


Core Concepts
Two cyano derivatives of the polycyclic aromatic hydrocarbon acenaphthylene, 1-cyanoacenaphthylene and 5-cyanoacenaphthylene, have been detected for the first time in the interstellar medium, specifically in the dark cloud TMC-1.
Abstract

The QUIJOTE ultrasensitive line survey has enabled the discovery of two cyano derivatives of the polycyclic aromatic hydrocarbon (PAH) acenaphthylene (C12H8) in the dark cloud TMC-1.

The first series of lines, labeled B429, corresponds to 1-cyanoacenaphthylene. A total of 173 rotational transitions with J up to 46 and Ka up to 9 have been detected and assigned, corresponding to 107 independent frequencies.

The second series, labeled B444, is assigned to 5-cyanoacenaphthylene. This identification is based on 56 individual lines, corresponding to 117 rotational transitions with J up to 40 and Ka up to 8.

The identification of the carriers was achieved through a careful analysis of the derived rotational constants, which indicate that the species are planar, allowing the authors to discard derivatives of non-planar molecules such as fluorene and acenaphthene. Quantum chemical calculations and subsequent chemical synthesis of these molecules, as well as the observation of their rotational transitions in the laboratory, unequivocally support the identifications.

The authors also confirm, through a robust line-by-line detection, the previous claimed detection of 1- and 2-cyanonaphthalene, which were obtained through statistical stacking techniques.

The column densities of 1- and 5-cyanoacenaphthylene are estimated to be (9.5 ± 0.9) × 10^11 cm^-2, while those of 1- and 2-cyanonaphthalene are (5.5 ± 0.5) × 10^11 cm^-2. This suggests that acenaphthylene could be a factor of 1.7 more abundant than naphthalene in TMC-1.

These results support a scenario in which polycyclic aromatic hydrocarbons grow in cold dark clouds based on the formation of fused five- and six-membered carbon rings.

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Stats
The derived rotational constants for 1-cyanoacenaphthylene (B429) are: A = 1271.64 MHz, B = 647.27938(19) MHz, C = 429.06155(14) MHz. The derived rotational constants for 5-cyanoacenaphthylene (B444) are: A = 1246.5874(75) MHz, B = 690.1787(24) MHz, C = 444.29815(32) MHz.
Quotes
"The derived rotational constants indicate that the species are planar; this allows us to discard derivatives of fluorene and acenaphthene, which are non-planar species." "Quantum chemical calculations and subsequent chemical synthesis of these molecules, as well as the observation of their rotational transitions in the laboratory, unequivocally support our identifications."

Deeper Inquiries

What other polycyclic aromatic hydrocarbon species might be present in TMC-1 and how could their detection provide further insights into the chemical growth of these complex molecules in cold dark clouds?

The presence of other polycyclic aromatic hydrocarbons (PAHs) in TMC-1 could include species such as phenanthrene, anthracene, and pyrene, among others. The detection of these PAHs would be significant as it would provide a more comprehensive understanding of the chemical pathways leading to the formation of complex organic molecules in cold dark clouds. In particular, the identification of larger PAHs could indicate a bottom-up growth mechanism, where smaller hydrocarbons and radicals combine to form larger structures. This is supported by the detection of cyano derivatives, which suggests that PAHs are not only formed but also modified through chemical reactions involving cyano groups. The presence of various PAHs could also reveal the relative abundances of different species, shedding light on the specific conditions and chemical processes occurring in TMC-1. For instance, the ratios of different PAHs could inform us about the efficiency of various formation pathways, such as radical recombination or ion-neutral reactions, which are crucial for understanding the astrochemical evolution of these regions.

How do the formation pathways and abundances of 1-cyanoacenaphthylene and 5-cyanoacenaphthylene compare to those of other cyanide derivatives of PAHs, such as cyanonaphthalenes, and what implications does this have for our understanding of PAH chemistry in the interstellar medium?

The formation pathways of 1-cyanoacenaphthylene and 5-cyanoacenaphthylene are likely similar to those of other cyanide derivatives of PAHs, such as cyanonaphthalenes, in that they may involve the addition of cyano groups to existing PAH structures. However, the observed abundances indicate that cyanoacenaphthylenes are approximately 1.7 times more abundant than cyanonaphthalenes in TMC-1. This suggests that the growth of PAHs in cold dark clouds may favor larger structures, possibly due to the stability and reactivity of fused ring systems. The implications of these findings for our understanding of PAH chemistry in the interstellar medium are profound. They suggest that the chemical pathways leading to PAH formation are not only efficient but also selective, favoring the production of larger, more complex molecules. This could indicate a rich chemical environment in TMC-1 that supports the growth of PAHs through a series of reactions involving smaller hydrocarbons and radicals. Furthermore, the detection of these specific cyano derivatives highlights the importance of functionalization in PAH chemistry, which could influence their physical properties and reactivity in the interstellar medium.

Could the detection of these cyanoacenaphthylene isomers in other interstellar environments provide information about the physical and chemical conditions in those regions, and how they differ from the conditions in TMC-1?

Yes, the detection of cyanoacenaphthylene isomers in other interstellar environments could provide valuable insights into the physical and chemical conditions present in those regions. The presence of these molecules would suggest that similar chemical processes are occurring, indicating a commonality in the formation pathways of PAHs across different environments. However, variations in the abundance and types of detected PAHs could also reflect differences in the local conditions, such as temperature, density, and radiation fields. For instance, if cyanoacenaphthylenes were found to be more abundant in a region with higher temperatures, it could imply that thermal processes play a significant role in their formation. Conversely, a lower abundance in a different environment might suggest that the conditions are less conducive to PAH growth or that different chemical pathways are favored. Additionally, the detection of these isomers in various interstellar environments could help to map the distribution of PAHs and their derivatives throughout the galaxy, providing a clearer picture of the chemical evolution of the interstellar medium. This could ultimately enhance our understanding of the role of PAHs in astrochemistry, including their potential contributions to the formation of complex organic molecules and the origins of life.
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