Electron energy dependence of regioselective chloride ion loss from polychlorodibenzo-p-dioxins. Relationship between resonance electron energies andvirtual orbital energies

Regiospecifically chlorine-37-labeled polychlorodibenzo-p-dioxins (PCDDs) u ndergo electron energy-dependent regioselective chloride ion loss when anal yzed by electron capture negative ion mass spectrometry using an electron m onochromator to supply the slow monoenergetic electrons, Three negative ion resonances, produced with electrons of energies <0.5, similar to 1, and si milar to 4 eV are associated with the production of chloride ions from PCDD s. Negative ion resonances for the production of molecular ions of higher P CDDs were recorded with electrons of energies <0.2 eV. Dichlorodibenzo-p-di oxins and some trichlordibenzo-p-dioxins showed no molecular ions. A correl ation was found between the experimental electron attachment energies and t he virtual orbital energies calculated by modified density functional theor y methods using the B3LYP/D95// B3LYP/D95 level of theory. The results from these correlations strongly suggest that all negative ion-forming processe s for this class of compounds are initiated from pi* states. The loss of ch loride ion from transient negative PCDD ions requires pi*-sigma* orbital mi xing.