Substitution effects at α-position of divalent five-membered ring XC4H3M (M = C, Si and Ge)

► The ΔGs − t between singlet (s) and triplet (t) states of XC4H3C. ► X = -Cl > -Br > -CH3 > -H > -CF3 > -F > -NO2 > -OH > -NH2. The changes of ΔGs − t for XC4H3Si and XC4H3Ge. ► X = -NH2 > OH > F > Cl > Br > CH3 > H > CF3 > NO2.