Ab initio MO study on the S₁ ← S₀ origin transition energies of polychlorodibenzofurans (PCDFs)

The geometries and energies for the S₀ and S₁ states of dibenzofuran (DF) and all 135 PCDFs were obtained using Hartree-Fock (HF) and CI-Singles (CIS) methods. The transition energies determined from HF and CIS energies were corrected for electron correlation. If the electron correlation energies for the S₀ and the S₁ state are calculated using a second-order Møller-Plesset perturbation (MP2) and a perturbative correction to CIS (termed CIS(D)) method, the S₁ ← S₀ 0-0 transition energies are given with an error of 15.4-22.6%. Electron correlation corrections to the transition energies were determined from a selected set of experimental transition energies on the assumption of the following additivity rule: the electron correlation energy of each state can be partitioned into contributions from the parent molecule and substituent chlorines. The transition energies after correction for electron correlation are in good agreement with available experimental data with an error of 0.2-3.5%. The validity of the additivity rule with respect to electron correlation energy is discussed in relation to the geometry of the molecule. The findings show that the additivity rule holds within an error of 0.4%. Vertical ionization potentials, which are useful in REMPI spectral studies, were calculated using Koopmans' theorem. The results confirmed that the S₁ ← S₀ origin transition energy is inversely proportional to the number of chlorine atoms n_Cl but the ionization potential is directly proportional to n_Cl.