The failure of UMP2 on the keto-enol tautomerization of β-radical compounds: The effect of spin contamination

► We examine the relative stability of α-, β- and γ-ketonic and enolic radicals. ► In α- and γ-radicals, UMP2 and DFT schemes all favor ketonic form. ► In β-radicals, the UMP2 favors ketonic radical. ► In β-radicals, DFT and CCSD(T) schemes prefer enolic ones. ► The failure of UMP2 comes from the spin contamination problem for enolic form.