THE IONIZATION ENERGIES OF POLYCHLORINATED DIBENZO-P-DIOXINS - NEW EXPERIMENTAL RESULTS AND THEORETICAL-STUDIES

In this work we examine the effect of successive introductions of chlo rine substituents on the ionization energy (IE) of dibenzo-p-dioxin. T he ionization energies of dibenzo-p-dioxin (7.598 +/- 0.002 eV) and 2- monochlorodibenzo-p-dioxin (7.707 +/- 0.002 eV) are obtained using the method of resonance-enhanced two-color two-photon ionization (REMPI) in a cold molecular jet combined with time-of-flight (TOF) mass spectr ometry. They are compared with our recent results for 2,8-dibenzo-p-di oxin (Th.L. Grebner, H.J. Neusser and B. Ernstberger, Int. J. Mass Spe ctrom. Ion Processes, 136 (1994) 101) and with theoretical values obta ined with the AM1 Hamiltonian. The experimental as well as the theoret ical results show in a consistent way that the ionization energy incre ases by approximately 100 meV for each additional chlorine substituent up to 2,8-dichlorodibenzo-p-dioxin. For more highly chlorinated conge ners of dibenzo-p-dioxin a theoretical prediction of the ionization en ergies is presented. The limit for one-color resonance-enhanced two-ph oton ionization of 2,3,7,8-tetrachlorodibenzo-p-dioxin is estimated by comparison of the results of our IE investigation, spectroscopic data obtained in a supersonic jet and conventional UV/VIS solution spectra .