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

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.