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

Abstract

The identification of individual molecular species in the interstellar medium (ISM) via radio astronomical observations relies on precise laboratory measurement of the species' rotational spectrum. Following the detections of benzonitrile and ethynylbenzene in the ISM, we perform a search for the disubstituted benzene derivatives that feature both a nitrile and an ethynyl substituent: the ortho (o), meta (m), and para (p) isomers of ethynylbenzonitrile (EBN). Having previously been measured in the millimeter-wave regime, we extend the rotational spectroscopy of the isomeric family down to the centimeter-wave regime where hyperfine splitting due to the electric quadrupole moment of the ¹⁴N atom can be resolved. Transitions of the singly substituted ¹³C and ¹⁵N isotopologues of the o- and p-EBN isomers are observed in their respective spectra, and the rotational constants of these isotopologues are also determined. Experimental ground state structural parameters are determined for o- and p-EBN with the rotational constants of the observed isotopologues, and their geometries are compared to that of benzene to determine how the substituents distort the aromatic ring. With the hyperfine splitting characterized, the rotational spectra are compared to the narrow emission lines observed toward the cold molecular cloud TMC-1. No individual transitions are identified in the observational data, and upper limits on the column densities of the three isomers are derived from the observations. Rate coefficients and product branching ratios are theoretically estimated for the formation of the EBN isomers from both benzonitrile and ethynylbenzene via CCH and CN addition, respectively. Based on these formation rates, and the observed column densities of other substituted aromatic species, the column densities of each of the EBN isomers are estimated to be less than an order of magnitude below their computed upper limits.