Centimeter-Wave Rotational Spectroscopy of Ethynylbenzonitriles: Structural Analysis and Astronomical Search

Area of Science:

• Astrochemistry
• Radio Astronomy
• Quantum Chemistry

Background:

• Interstellar medium (ISM) molecular identification relies on laboratory rotational spectra.
• Benzonitrile and ethynylbenzene have been detected in the ISM.
• Disubstituted benzene derivatives with nitrile and ethynyl groups are potential interstellar molecules.

Purpose of the Study:

• Search for ortho (o), meta (m), and para (p) isomers of ethynylbenzonitrile (EBN) in the ISM.
• Extend rotational spectroscopy of EBN isomers to the centimeter-wave regime.
• Determine structural parameters and compare geometries of EBN isomers to benzene.

Main Methods:

• Extended rotational spectroscopy of EBN isomers down to the centimeter-wave regime.
• Observed transitions of ¹³C and ¹⁵N isotopologues of o- and p-EBN.
• Determined experimental ground state structural parameters for o- and p-EBN.
• Compared laboratory spectra with observational data from the cold molecular cloud TMC-1.
• Theoretically estimated formation rates and product branching ratios for EBN isomers.

Main Results:

• Resolved hyperfine splitting due to ¹⁴N electric quadrupole moment in EBN spectra.
• Determined rotational constants for ¹³C and ¹⁵N isotopologues of o- and p-EBN.
• No EBN isomer transitions were identified in TMC-1 observations, leading to upper limits on column densities.
• Estimated column densities of EBN isomers are close to derived upper limits.

Conclusions:

• The study provides crucial spectroscopic data for potential interstellar EBN detection.
• While not yet detected, EBN isomers are theoretically predicted to be near detectable abundances in TMC-1.
• Further observations and refined theoretical models are needed to confirm the presence of EBN isomers in the ISM.