Effects of alkyl substitution on the energetics of enolate anions and radicals

The photoelectron spectra of the structural isomers of the three- and four-carbon enolate anions, n-C₃H₅O^-, i-C₃H₅O^-, n-C₄H₇O^-, s-C₄H₇O^-, and i-C₄H₇O^- have been measured at 355 nm. Both the X(²A″) ground and A(²A′) first excited states of the corresponding radicals were accessed from the X(¹A′) ground state of the enolate anions. The separation energies of the ground and first excited states (T₀) were determined: T₀[(E)-n-C₃H₅O] = 1.19 ± 0.02 eV, T₀[(Z)-n-C₃H₅O] = 0.99 ± 0.02 eV, T₀[i-C₃H₅O] = 1.01 ± 0.02 eV, T₀[n-C₄H₇O] = 1.19 ± 0.02 eV, T₀[(2,3)-s-C₄H₇O] = 1.25 ± 0.02 eV, T₀[(1,2)-s-C₄H₇O] = 0.98 ± 0.02 eV, and T₀[i-C₄H₇O] = 1.36 ± 0.02 eV. The effects of alkyl substitution on the vibronic structure and energetics previously observed in the vinoxy radical are discussed. The X(¹A′)-X(²A″) relative stability is strongly influenced by substitution whereas the X(¹A′)-A(²A′) relative stability remains nearly constant for all of the observed structural isomers. Alkyl substitution at the carbonyl carbon affects vibronic structure more profoundly than the energetics, while the converse is observed upon alkyl substitution at the α carbon.