Muonium addition to the C=S sulfur in the conformationally regulated thiobenzophenone

Abstract

The C=S units can be functional for chemical processes by receiving radical species, and tuning the structures of thioketones is a promising approach to novel materials that 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 regioselective addition of muonium (muoniation) at the C=S sulfur atoms, 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 altered the reactivity of thiobenzophenone toward muonium potently due to the deformed conformation of thiobenzophenone. The findings in this work should be informative for developing novel functional thioketones and related molecules that tame paramagnetic radicals, as well as for understanding the chemistry of medium-sized cyclic molecular skeletons.