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 chlorine substituents on the ionization energy (IE) of dibenzo-p-dioxin. The 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 spectrometry. They are compared with our recent results for 2,8-dibenzo-p-dioxin (Th.L. Grebner, H.J. Neusser and B. Ernstberger, Int. J. Mass Spectrom. Ion Processes, 136 (1994) 101) and with theoretical values obtained with the AM1 Hamiltonian. The experimental as well as the theoretical results show in a consistent way that the ionization energy increases by approximately 100 meV for each additional chlorine substituent up to 2,8-dichlorodibenzo-p-dioxin. For more highly chlorinated congeners of dibenzo-p-dioxin a theoretical prediction of the ionization energies is presented. The limit for one-color resonance-enhanced two-photon 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.