Configuration interaction and density functional study of the influence of lithium cation complexation on vertical and adiabatic excitation energies of enones

Abstract

The changes in the excited state energies of representative cyclic enones (cyclopentenone and cyclohexenone) induced by lithium ion coordination have been examined using ab initio and DFT methods. Quantitative estimates of the vertical triplet state energies were obtained using configuration interaction calculations at the CIS and CIS(D) levels with the 6-31+G(d) basis. Inclusion of perturbative doubles corrections has a marked effect on the relative energies of the n-π^∗ and π-π^∗ triplet states. At both CI and CIS(D) levels, lithium complexation is predicted to raise the energy of the n-π triplet state much more than the π-π^∗ triplet. The trends obtained at the CIS(D) level are reproduced using B3LYP/6-31+G(d) calculations. Adiabatic excitation energies were also computed by carrying out geometry optimization of the triplet states at the B3LYP level. While the separation between the geometry optimized n-π^∗ and π-π^∗ triplet states is very small for the parent enones, the π-π^∗ triplet is clearly favored in the lithium complexes. These results suggest the possibility of reversing the reactive photoexcited state in enones through cation complexation. The conclusions provide a rationale for interesting variations in product distributions observed for enones in cation exchanged zeolites.