Rotational spectroscopy of imidazole: Accurate spectroscopic information for three vibrationally excited states and the heavy-atom isotopologues up to 295 GHz

•Imidazole was studied between 2–295 GHz; hydantoin was studied between 18–26 GHz.•Three vibrationally excited states of imidazole were observed and assigned.•Isotopologue analysis allowed gas-phase structure determination for both molecules.•Imidazole, hydantoin, and imidazolidine structures and vibrations are compared.•Precise rotational constants and line lists are delivered for all assigned species.