Open Access Article
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Searches for bridged bicyclic molecules in Space---Norbornadiene and its cyano derivatives

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Marie-Aline Martin-Drumel , Jean-Thibaut Spaniol , Helen Hölzel , Marcelino Agundez , Jose Cernicharo , Kasper Moth-Poulsen and Ugo Jacovella

Received NaNth , Accepted 14th February 2023

First published on 16th February 2023


The norbornadiene (NBD) molecule, C8H7, owes its fame to its remarkable photoswitching properties promising for molecular solar-thermal energy storage systems. Besides this photochemical interest, NBD is a rather unreactive species within astrophysical conditions and it should exhibit large photostability, properties that might also place this molecule as an important constituant of the interstellar medium (ISM)---especially in environments that are well shielded from short wavelength radiation such as dense molecular clouds. It is thus conceivable that, once formed, NBD can survive in dense molecular clouds and act as a carbon sink. Following the recent interstellar detections of large hydrocarbons, including several cyano containing ones, in the dense molecular clouds TMC-1, it is thus logical to consider searching for the NBD---which presents a shallow but non-zero permanent electric dipole moment (0.06 D)---as well as for its mono and dicyano substituted compounds, referred as CN-NBD and DCN-NBD, respectively. The pure rotational spectra of NBD, CN-NBD, and DCN-NBD have been measured at 300 K in the 75--110 GHz range using a chirped-pulse Fourier transform millimetre-wave spectrometer. Of the three species, only NBD was previously studied at high resolution in the microwave domain. From the present measurements, the derived spectroscopic constants enable to predict the spectra of all three species at various rotational temperature (up to 300 K) in the spectral range mapped at high resolution by current radio observatories. Unsuccessful searches for these molecules were conducted toward TMC-1 using the QUIJOTE survey, carried out at the Yebes telescope, allowing to derive upper limits to the column densities of 1.6 × 1014 cm-2, 4.9 × 1010 cm-2, and 2.9 × 1010 cm-2 for NBD, CN-NBD, and DCN-NBD, respectively. Using CN-NBD and cyano-indene as proxy for the corresponding bare hydrocarbons, this indicates that---if present in TMC-1---NBD would be at least four times less abundant than indene.