Relationships were found between experimentally measured molecular radical anion abundances and calculated lowest unoccupied molecular orbital energies (epsilon LUMO) for polychlorodibenzofurans and polychlorodibenzo-p-dioxins. Anion abundances were measured using standard mass spectrometric techniques, while epsilon LUMO were calculated by the 'Complete Neglect of Differential Overlap' method. Polychlorodibenzofurans with calculated epsilon LUMO greater than or equal to 1.6 eV show 0% molecular radical anion and those with epsilon LUMO less than or equal to 1.4 eV show greater than or equal to 80% molecular radical anion abundance. Similarly, the molecular radical anion is absent for polychlorodibenzo-p-dioxins with calculated epsilon LUMO greater than or equal to 2.0 eV. A trend towards greater molecular radical ion relative abundance appears for 2.0 eV greater than or equal to epsilon LUMO greater than or equal to 1.0 eV and a maximum is reached around 1 eV, whereupon the molecular ion abundance diminishes with lower epsilon LUMO. B/E linked scan analysis indicates that chlorodioxins with epsilon LUMO less than 1 eV give increasing amounts of metastable anions.