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

Relative stabilities of α-, β- and γ-ketonic and enolic radicals have been examined using U/P/ROMP2 and UDFT methods. Our results show that in α- and γ-radicals, UMP2 and DFT schemes all favor ketonic form. However, in β-radicals, the UMP2 favors ketonic radical in keto–enol tautomerization of compound containing a radical at β position whereas DFT and CCSD(T) schemes prefer enolic ones. The failure of β-radicals for UMP2 method comes from the large effect of spin contamination for enolic form, and can be corrected by using ROMP2 and PMP2 methods.