We measured and analyzed the microwave (11-35 GHz) and millimeter-wave (85-750 GHz) rotational spectra of the normal isotopologue of cyclopropanone and nine additional isotopologues. The ability to perform experimental measurements is predicated on the synthesis of stable oligomers (or polymers) of cyclopropanone that release the monomeric form under vacuum to enable the investigation of cyclopropanone by gas-phase rotational spectroscopy. The spectral data provided in this work establish the foundation for radioastronomical searches for cyclopropanone. In particular, the new low-frequency microwave observations provide hyperfine-resolved rotational transitions that are necessary for radioastronomical observations at the low temperatures that characterize many cold interstellar molecular clouds. The extensive isotopic data set allows the first complete semiexperimental equilibrium (reSE) structure of cyclopropanone. The 10 isotopologues used in this structure determination provide 30 moments of inertia. As a result, the values of all six independent structural parameters are highly converged, establishing a new experimental benchmark for the structure of this archetypal organic molecule. The highly precise and accurate reSE structure was compared to a computed equilibrium (re) structure at the CCSD(T)/cc-pCV6Z level with additional corrections addressing finite basis set, higher-level electron correlation, and relativistic effects, as well as the diagonal Born-Oppenheimer correction. The computed re structure systematically deviates from the reSE structure, despite the high-level methodology utilized. This finding is interpreted as revealing a subtle, but real, challenge for theoretical chemistry in predicting molecular structure at the level of accuracy that is now attainable experimentally.
