CORRELATION BETWEEN ELECTRON-CAPTURE NEGATIVE CHEMICAL-IONIZATION MASS-SPECTROMETRIC FRAGMENTATION AND CALCULATED INTERNAL ENERGIES FOR POLYCHLORINATED-BIPHENYLS

Correlations between molecular structure and fragmentation observed in electron capture negative chemical ionization mass spectra (moderator gas = methane) of 49 selected tetrachlorinated, pentachlorinated, and hexachlorinated biphenyls have been investigated by using molecular m odeling. The semiempirical general molecular orbital program MOPAC was used to calculate molecular properties for biphenyl and the 209 polyc hlorinated biphenyls. The mass spectrometric ionization and fragmentat ion processes were found to be linked to the number of chlorine atoms present on the biphenyl, and to the number of those chlorine atoms in the ortho (2, 2', 6, and 6') positions. The intensity of molecular ion s increased with the number of chlorine atoms present, but this was co unteracted by enhanced fragmentation as the number of ortho position c hlorine atoms increased. The molecular parameters that were most close ly linked with the number of ortho chlorine atoms were the twist angle between the phenyl rings and the energy of the lowest unoccupied mole cular orbital (LUMO). It is suggested that fragmentation occurs when t he energy of the ionizing electron exceeds the energy difference betwe en the LUMO and LUMO + 1 orbitals.