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

Abstract

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.

Modulating Escape Channels of Cycloheptatrienyl Rhodium Carbenes To Form Semibullvalene

“…Unlike their reaction, however, questions about oxidation state are irrelevant with our [Rh 2 ] catalysts. The relative energies of barbaralones complexed to the gold­(I)-catalyst are qualitatively consistent with experimental and theoretical work by Alonso and Echavarren on bare barbaralones: in particular, the 1-phenyl-barbaralone ( Int O in Figure ) is the more thermodynamically stable isomer (here, by 1.1 kcal mol –1 ) …”

The eight structures of caffeic acid: a jet-cooled laser ablated rotational study

“…One unique aspect of rotational studies is that even very subtle changes in the molecular structure are translated into a distinctive array of spectroscopic constants. [29][30][31] Nevertheless, the major constraint of transferring caffeic acid to the gas phase is its solid nature (mp 213 °C) and its inherent vaporization problems. Nowadays, it has become possible to explore the architecture of solid biomolecules in the gas phase by techniques that couple laser ablation (LA), for transferring them intact into the gas phase, with high-resolution Fourier transform microwave (FTMW) techniques in a supersonic expansion.…”

Stochastic Bullvalene Architecture Modulates Structural Rigidity in π‐Rich Macromolecules

“…[26] Stochastic kinks introduced by an ensemble of regioisomers could allow for tuning of thermal transitions in π-rich materials, but synthetic implementation would be cumbersome. This paradigm could instead be realized by invoking a single molecule capable of "shape-shifting" [27][28][29][30][31][32] amongst different valence isomers to provide the stability of a rigid polymer while also increasing structural flexibility to approximate the impact of an alkyl spacer. Bullvalene [27,29,33,34] (Figure 1B) is the ideal molecule to test this strategy; bullvalene's unique cage structure allows for low barrier sigmatropic rearrangements (i.e., Hardy-Cope rearrangements, [35] E a = 11 kcal mol À 1 ) [28] to access an ensemble of valence isomer (> 1 million) geometries.…”