INFLUENCE OF CHLORINE SUBSTITUENT POSITIONS ON THE MOLECULAR-STRUCTURE AND THE TORSIONAL POTENTIALS OF DICHLORINATED BIPHENYLS - R2PI SPECTRA OF THE 1ST SINGLET TRANSITION AND AM1 CALCULATIONS

Resonance enhanced two-photon ionization (R2PI) spectra of biphenyl an d four dichlorinated biphenyl derivatives (2,6-,2,2'-,4,4'- and 2,4'-d ichlrobiphenyl) cooled in a supersonic jet expansion have been measure d in the region of the S1 <-- S0 transition (34 500-36 900 cm-1). The biphenyl derivatives show different torsional angles between the two p henyl rings. The degree of torsion depends on the positions of the sub stitutents due to differing steric and electronic effects on the biary l chromophore. Furthermore the dihedral angle changes upon electronic excitation. Therefore the electronic spectra of all investigated compo unds show a vibronic substructure, dominated by Franck-Condon induced progressions of the torsional mode. Nevertheless the four dichlorobiph enyl 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 st ructure of the R2PI spectra has been interpreted with the help of thes e calculations. In addition, the use of R2PI as an analytical tool is demonstrated. R2PI of jet-cooled aromatic molecules provides the possi bility of species and isomer selective ionization from a complex mixtu re. In the case of polychlorinated biphenyls (PCBs) an isomer selectiv e registration of the important toxic (coplanar) PCBs should be possib le.