A Hydrogen‐Bonded Organic Biradical in the Gas Phase

Persistent organic radicals from the TEMPO family such as the 4‐hydroxy derivative TEMPOL can interact non‐covalently. The resulting reversible biradicals have interesting electronic features and applications. Very little is known about their structural and vibrational properties under conditions, which are amenable to rigorous theoretical description, at low temperature in vacuum. The OH stretching vibrations in complexes of TEMPOL with itself and for comparison with TEMPO and with cyclohexanol are therefore investigated in supersonic jet expansions. They turn out to be sensitive to the hydrogen‐bonded OH conformation, the coordination face of the TEMPO moiety, and the bulkiness of the OH carrier. Quantum‐chemical calculations at harmonically evaluated B3LYP as well as CCSD(T) level help in spectrally assigning the complexes, but they also reveal some qualitative DFT limitations even for the constituent radicals and their closed‐shell analogues, as is demonstrated by comparison to experiment. This proof‐of‐concept should encourage the application of other spectroscopic techniques to supramolecular biradicals in adiabatic expansions or other suitable molecular isolation, which address their electronic properties more directly.