Theoretical microwave spectra of interstellar nitrogen-containing PAHs

Area of Science:

• Astrochemistry
• Computational Chemistry
• Molecular Spectroscopy

Background:

• Naphthalene (C₁₀H₈) was recently detected in cyano-substituted polycyclic aromatic hydrocarbon (CN-PAH) form in the Taurus Molecular Cloud (TMC-1).
• This discovery motivates the search for other nitrogen-containing naphthalenes in similar cold, dark interstellar environments.
• Laboratory characterization of these molecules is challenging.

Purpose of the Study:

• To report theoretical microwave spectra for all N-substituted naphthalene isomers.
• To provide accurate rotational data to aid laboratory experiments and astronomical searches.
• To identify promising candidates for detection in cold molecular clouds.

Main Methods:

• Density Functional Theory (DFT) calculations were employed.
• Spectroscopic constants and rotational spectra, including hyperfine splitting, were calculated.
• Simulations focused on conditions relevant to cold molecular clouds (approx. 5 K).

Main Results:

• Theoretical microwave spectra were generated for all N-naphthalene isomers.
• The strongest transitions for these species are predicted around centimeter wavelengths.
• These wavelengths are typical for polycyclic aromatic hydrocarbon-related species in dark molecular clouds.

Conclusions:

• N-naphthalene species are promising candidates for detection in cold interstellar clouds.
• The provided accurate rotational data can guide future laboratory and observational efforts.
• This theoretical study facilitates the search for complex nitrogen-containing organic molecules in space.