The structure of electronically excited α,β-unsaturated lactones

better knowledge of the structure of the electronically excited state of substrates is indispensable for the understanding and optimization of photochemical reactions. For this study, triplet energies of a variety of α,β-unsaturated γ-lactones (furanones) as well as the structures of the vibrationally relaxed triplet state (T1) have been determined using ab initio coupled-cluster (CCSD) method and/or density functional theory (DFT) calculation. A twist of the original planar structure around C = C bond is found in the relaxed triplet state, π-π*. In the 5-membered ring of furanones the contribution of this mode is limited and the pyramidalization in the C4 position also contributes to the stabilization. The contribution of each stabilization mode is characterized by the dihedral angles and the Mulliken atomic spin densities. The substituent effect on the pyramidalization and the spin density distribution in the C4 and in the C5 position are reported. Depending on the substitution in the C4 position, the orientation of the pyramidalization is either favored syn or anti with respect of the hydroxyl substituent in the C5 position.