Influence of chlorine substituent positions on the molecular structure and the torsional potentials of dichlorinated biphenyls: R2PI spectra of the first singlet transition and AM1 calculations

Abstract

Abstract Resonance enhanced two-photon ionization (R2PI) spectra of biphenyl and four dichlorinated biphenyl derivatives (2,6-, 2,2-, 4,4- and 2,4-dichlorobiphenyl) cooled in a supersonic jet expansion have been measured in the region of the S 1 ← S 0 transition (34 500–36 900 cm −1 ). The biphenyl derivatives show different torsional angles between the two phenyl rings. The degree of torsion depends on the positions of the substituents due to differing steric and electronic effects on the biaryl chromophore. Furthermore the dihedral angle changes upon electronic excitation. Therefore the electronic spectra of all investigated compounds show a vibronic substructure, dominated by Franck—Condon induced progression of the torsional mode. Nevertheless the four dichlorobiphenyl spectra differ both in energetic locations and vibronic structure. The molecular structures and torsional potentials have been studied by molecular orbital calculations using the AM1 method. The complex structure of the R2PI spectra has been interpreted with the help of these calculations. In addition, the use of R2PI as an analytical tool is demonstrated. R2PI of jet-cooled aromatic molecules provides the possibility of species and isomer selective ionization from a complex mixture. In the case of polychlorinated biphenyls (PCBs) an isomer selective registration of the important toxic (coplanar) PCBs should be possible.