Conformational changes and S1 ← S0 origin transition energies: Polychlorinated biphenyls (PCBs)

Geometries and S1 ← S0 origin transition energies for biphenyl, deuterated biphenyl, and 24 PCBs were obtained using a Hartree-Fock (HF), a single-excitation configuration interaction (CIS), and a related CI method with doubles correction (CIS(D)). The HF and CIS geometries for biphenyl are in fairly good agreement with those obtained by a complete active space self-consistent field (CASSCF) method. Calculated results show that, in both S0 and S1 states of PCBs, the torsional angles and the inter-ring bond lengths can be classified according to the number of ortho-chlorine atoms, no-Cl. This suggests that the difference in electronic energy between the S0 and the S1 state also can be specified by no-Cl. The HF and CIS energies were corrected for electron correlation on the assumption that the electron correlation energy can be partitioned into constituent contributions. Calculated S1 ← S0 origin transition energies agree with experimental values within an error of 2%. This level of accuracy is comparable to that of the S1 ← S0 origin transition energy for biphenyl calculated using the CASSCF method. The correction for electron correlation energy is largely due to contributions of the parent molecule and ortho-chlorine atoms. The present results will facilitate the analysis of electronic spectra of PCBs.