Chirped-Pulse Fourier Transform Millimeter-Wave Spectroscopy of Furan, Isotopologues, and Vibrational Excited States

Recent astronomical detections of
several five- and six-membered
carbon rings in the interstellar medium (ISM) have expanded our understanding
of the chemistry operative in diverse astrophysical environments.
These discoveries prompt new interest in astronomical searches for
the more chemically complex heterocycles, and laboratory studies of
the rotational spectra of such molecules are a crucial first step.
We present a new investigation of the rotational spectrum of the aromatic
heterocycle furan (C4H4O) in selected bands
from 75 to 295 GHz by chirped-pulse Fourier transform millimeter-wave
spectroscopy. The improved determination of molecular constants is
suitable for computing rest frequencies for astronomical searches
throughout the millimeter-wave region. We also present new rotational
analyses of the two singly substituted carbon-13 isotopomers and the
two lowest vibrational excited states, ν11 and ν14. Using the second data release from the GOTHAM spectral
line survey of the starless cold core Taurus Molecular Cloud 1, we
determine a new upper limit on the abundance of furan in this source,
further demonstrating the peculiarly low abundance of heterocycles
relative to pure hydrocarbon cycles in the ISM.