Muonium Addition to a peri‐Trifluoromethylated 9‐Phosphaanthracene Producing a High‐Energy Paramagnetic π‐Conjugated Fused Heterocycle

Abstract

AbstractIn this communication, we report muon spin rotation/resonance (μSR) studies for understanding radical reactivity of 10‐mesityl‐1,8‐bis(trifluoromethyl)‐9‐phosphaanthracene. Transverse‐field muon spin rotation (TF‐μSR) and muon avoided level‐crossing resonance (μLCR) measurements successfully visualized a paramagnetic species produced by regioselective addition of muonium (Mu) to the skeletal phosphorus atom. Density functional theory (DFT) calculations for the P‐muoniation product suggested two possible isomers. Whereas the most stable isomer including the envelope‐type phosphorus heterocycle shows considerably different hyperfine coupling constants (hfcs) from those of the TF‐μSR and μLCR, the metastable structure accompanying the almost planar tricyclic π‐conjugated skeleton could simulate the experimentally determined hfcs. The metastable planar π‐conjugated paramagnetic tricyclic‐fused skeleton is promoted by the larger zero‐point energy due to the light muon (μ+), one ninth of the proton mass.