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 valence tautomers have been isolated in a supersonic expansion for the first time and successfully characterized by high-resolution rotational spectroscopy. This work provides irrefutable experimental evidence of the [3,3]-rearrangement in barbaralones and highlights the use of rotational spectroscopy to analyze shape-shifting mixtures. Moreover, this observation opens the window toward the characterization of new fluxional systems in the isolation conditions of the gas phase and should serve as a reference point in the general understanding of valence tautomerism.