Microwave rotational spectrum and ab initio equilibrium structure of fumaric acid: Anharmonicity bridging the molecular characterizations

► Fumaric Acid is observed by supersonic jet expansion microwave spectroscopy. ► Spectroscopic constants and electric dipole moments are obtained at high accuracy. ► Spectroscopic and ab initio data agree on a low-energy s-cis,s-trans conformer. ► Experiment and theory are compatible when anharmonic effects are taken into account.