The psi' - eta'$_{c}$ hyperfine splitting

The hyperfine splitting of the radially excited states psi ' and eta /sub 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( psi ')-M( eta /sub 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.