The ψ′-η′c hyperfine splitting

The hyperfine splitting of the radially excited states ψ′ and η′_c is studied within the framework of potential models. Some rigorous results are derived and some phenomenological approaches are commented on. The recent experimental result M(ψ′)?M(η′_c) = 93 MeV favours very short-range spin-spin forces and seems, on the other hand, difficult to accommodate if there is a sizeable effect of virtual D meson pairs on the charmonium mass spectrum.