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Dibenzo-p-dioxin. An ab initio CASSCF/CASPT2 Study of the pi-pi* and n-pi* Valence Excited States.

The pi-pi* and n-pi* valence excited states of dibenzo-p-dioxin (DD) were studied via the complete active space SCF and multiconfigurational second order perturbation theory employing the cc-pVDZ basis set and the full pi-electron active spaces of 16 electrons in 14 active orbitals. The geometry and harmonic vibrational wavenumbers of the ground state correlate well with the experimental and other theoretical data. In particular significant improvements over previously reported theoretical results are observed for the excitation energies. All of the pi-pi* excited states exhibit planar D2h minima. Thus no evidence was found for a C2v butterfly-like relaxation, although the wavenumbers of the b3u butterfly flapping mode proved exceedingly low in both, the ground S0(1Ag) and the lowest dipole allowed excited S1(1B2u) state. The calculations of oscillator strengths established the 21B2u <- 11Ag and 21B1u <- 11Ag transitions as by far the most intense, whereas the only allowed of the n-pi* transitions (1B3u) should possess only a modest intensity. Studies into dependence of the oscillator strengths on the extent of the butterfly-like folding showed that the electronic spectrum is more consistent with a folded equilibrium geometry assumed by DD in solution.

dibenzo-p-dioxin; ab initio methods; electronic spectrum; valence excited state; CASSCF; CASPT2

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