Shape‐Shifting Molecules: Unveiling the Valence Tautomerism Phenomena in Bare Barbaralones

We report a state-of-the-art spectroscopic study of an archetypical barbaralone, conclusively revealing the valence tautomerism phenomena for this bistable molecular system. The two distinct 1- and 5-substituted tautomers have been isolated in supersonic expansion first time successfully characterized by high-resolution rotational spectroscopy. This work provides irrefutable experimental evidence [3,3]-rearrangement barbaralones highlights use spectroscopy to analyze shape-shifting mixtures. Moreover, observation opens window toward characterization new fluxional systems isolation conditions gas phase should serve as reference point general understanding tautomerism.