Muonium addition to the CS sulfur in conformationally regulated thiobenzophenone.

CS units can have functionality for chemical processes by receiving radical species, and tuning the structures of thioketones is a promising approach for obtaining novel materials that can tame radical species. In this paper, we designed conformationally regulated thiobenzophenones by bridging the benzene rings with alkyl chains and employed them for directly observing the addition of muonium, a light isotope of hydrogen, in the solid state. The cyclic thioketones permitted the regioselective addition of muonium (muoniation) at the CS sulfur atom, and the corresponding C-centered radicals were characterised by muon spin resonance (µSR) spectroscopy. The 9-membered cyclic skeleton was advantageous for locking the intrinsic conformation of thiobenzophenone, and the CS group received muonium with considerable efficiency, even at low temperatures. On the other hand, the 7- and 8-membered cyclic skeletons potently altered the reactivity of thiobenzophenone toward muonium due to the deformed conformation of thiobenzophenone. The findings in this work should be informative for developing novel functional thioketones and related molecules that can tame paramagnetic radicals, as well as for understanding the chemistry of medium-sized cyclic molecular skeletons.