The electronic excited states of a model organic endoperoxide: A comparison of TD-DFT and ab initio methods

The vertical excited spectrum of a model endoperoxide (cyclohexadieneendoperoxide) has been calculated using time dependent density functional theory (TD-DFT), resolution of the identity second order approximate coupled-cluster theory (RI-CC2), multiconfigurational complete active space self consistent field (CASSCF) and second order multi-state perturbation theory (MS-CASPT2). All theoretical methods predict the charge transfer π OO ∗ → π CC ∗ , and the π OO ∗ → σ OO ∗ excitation to be the lowest absorbing excited states. CASSCF optimized geometries for these states provide some hints about the photodissociation mechanisms as well as the emission spectrum of the molecule.