Conformational changes and S 1 ← S 0 origin transition energies: Polychlorinated biphenyls (PCBs)

Abstract

Geometries and S 1 ← S 0 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 S 0 and S 1 states of PCBs, the torsional angles and the inter-ring bond lengths can be classified according to the number of ortho-chlorine atoms, n o-Cl . This suggests that the difference in electronic energy between the S 0 and the S 1 state also can be specified by n o-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 S 1 ← S 0 origin transition energies agree with experimental values within an error of 2%. This level of accuracy is comparable to that of the S 1 ← S 0 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.