Investigation Of Pure Rotational Spectroscopy Of Ethynylbenzonitrile Isomers Using Chirped-pulse W-band Spectroscopy

\begin{wrapfigure}{r}{0pt} \includegraphics[scale=0.14]{ETB234.eps} \end{wrapfigure} Simple aromatic molecules may be precursors for polycyclic aromatic hydrocarbons in space, and some of the simplest ones are now detected in the earliest stages of star formation [1]. Evidence of benzonitrile (C$_6$H$_5$--CN) in the interstellar medium [2] questions the presence of related aromatic nitriles and their ring-chain derivatives. In light of previous work [3] on phenylpropiolonitrile (C$_6$H$_5$--C$_3$N), we are investigating, by laboratory high-resolution studies, its ethynylbenzonitrile (HCC--C$_6$H$_4$--CN) derivatives where a --CN and a --CCH groups lie in ortho (2-ETB), meta (3-ETB) or para (4-ETB) positions. \smallskip The pure rotational spectrum of these compounds has been recorded at room temperature in the millimeter-wave domain using a chirped-pulse W-band (75--110 GHz) spectrometer. To facilitate spectral assignments, quantum chemical calculations have been performed using density functional theory at the $\omega$B97X-D/cc-pVQZ level of theory (geometry optimization, harmonic frequencies). We will report a description of the experimental set-up and of our assignment procedure. \bigskip \noindent [1] A.Burkhardt et al., Nature Astronomy 5, 181-187 (2021) \noindent [2] McGuire et al., Science 359, 202-205 (2018) \noindent [3] Z.Buchanan et al., Journal of Molecular Spectroscopy, in press (2021) \bigskip \noinden

Technical Enhancements Of A Submillimeter-wave Spectrometer: Laboratory Detection Of New Lines Of Methanol Radical Derivatives

Detection of radicals in the interstellar medium, such as hydroxymethyl (CH$_2$OH) and methoxy (CH$_3$O), is a highly interesting tool for better understanding the formation of commonly observed complex organic molecules such as glycolaldehyde, ethanol, ethylene glycol, and dimethyl ether. In this context, improving the predictions of astronomical lines with a well-defined model, based on laboratory measurements, becomes decisive. In an attempt to enrich models of CH$_2$OH [1] and CH$_3$O with new frequencies, we have re-investigated their pure rotational spectrum in the millimeter-wave domain. Both radicals were produced at room temperature by fluorine abstraction of hydrogene from methanol. We will report the technical improvements that have been made to increase both the sensitivity and the signal-to-noise ratio of our experimental set-up [1]. We suceeded to increase synthesis yield of both radicals by multiplying fluorine injections and we also further improved our optical set-up, now using a double passage of the beam in the cell. Finally, we wrapped an induction coil around the cell to create a magnetic field, in addition to the usual frequency modulation, allowing us to operate using double modulation detection scheme thus making us sensitive to species affected by Zeeman splitting. Both signals (single and double modulation) are recovered separately and displayed simultaneously on the measurement software. A strength of the double modulation scheme is that only lines arising from open-shell molecules, such as CH$_2$OH and CH$_3$O, are visible over a flat baseline (no residual Fabry-Perot interference fringes). These improvements significantly increase our sensitivity to short-lifetime species and grant a powerful tool for distinguishing radicals from stable molecules. \bigskip \noindent [1] O. Chitarra et al., Astronomy and Astrophysics (2020) 644, A 12

Searches for bridged bicyclic molecules in space-norbornadiene and its cyano derivatives

The norbornadiene (NBD) molecule, C7H8, owes its fame to its remarkable photoswitching properties that are 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 high photostability, properties that might also position this molecule as an important constituent 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 cloud TMC-1, it is thus logical to consider searching for 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 to 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 prediction of the spectra of all three species at various rotational temperatures (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 derivation of the upper limits to the column densities of 1.6 x 10(14) cm(-2), 4.9 x 10(10) cm(-2), and 2.9 x 10(10) cm(-2) for NBD, CN-NBD, and DCN-NBD, respectively. Using CN-NBD and cyano-indene as proxies for the corresponding bare hydrocarbons, this indicates that-if present in TMC-1-NBD would be at least four times less abundant than indene