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.