Correlation between electron capture negative chemical ionization mass spectrometric fragmentation and calculated internal energies for polychlorinated biphenyls

Correlationbse tween 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 modeling. The semiempirical general molecular orbital program MOPAC was used to calculate molecular properties for biphenyl and the 209 polychlorinated biphenyls. The mass spectrometric ionization and fragmentation 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 ions increased with the number of chlorine atoms present, but this was counteracted by enhanced fragmentation as the number of ortho position chlorine atoms increased. The molecular parameters that were most closely linked with the number of ortho chlorine atoms were the twist angle between the phenyl rings and the energy of the lowest unoccupied molecular orbital (LUMO). It is suggested that fragmentation occurs when the energy of the ionizing electron exceeds the energy difference between the LUMO and LlJMO + 1 orbitals.