The structure, vibrational frequencies, and excited states of 2, 3, 7, 8-tetrachloro-, 1, 4, 6, 9-tetrachloro-, and octachlorodibenzo-p-dioxin (2, 3, 7, 8-TCDD, 1, 4, 6, 9-TCDD, and OCDD) were studied via complete active space SCF followed by the multireference second-order perturbative approach (CASSCF/CASPT2), as well as the time-dependent density functional theory (TD-B3LYP). The cc-pVDZ basis set and the full -electron active spaces of 16 electrons in 14 active orbitals were employed. Whereas 2, 3, 7, 8-TCDD assumes a planar D2h minimum, 1, 4, 6, 9-TCDD and OCDD are slightly folded exhibiting the C2v symmetry. The extra stabilization due to the folding is very small. In all three isomers the highly intensive band system in the 200-240 nm region is dominated by the transitions to the 2 1B2u and 3 1B2u states with by far the largest oscillator strengths. The low-intensity absorptions in the 280-320 nm region can be attributed to the 1 1B2u 1 1Ag, with possible contribution of the vibronically allowed 2 1Ag 1 1Ag transition. Both CASPT2 and TD-B3LYP convincingly predict 1 3B3g to be the lowest lying triplet state, which contradicts the experimental assignments. Calculated harmonic wavenumbers and absorption spectra agree well with the experimental data, and are sufficiently distinct to allow for an unambiguous identification of the three isomers.

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