CORRELATION BETWEEN ELECTRON-CAPTURE NEGATIVE CHEMICAL-IONIZATION MASS-SPECTROMETRIC FRAGMENTATION AND CALCULATED INTERNAL ENERGIES FOR POLYCHLORINATED-BIPHENYLS
J. Greaves et al., CORRELATION BETWEEN ELECTRON-CAPTURE NEGATIVE CHEMICAL-IONIZATION MASS-SPECTROMETRIC FRAGMENTATION AND CALCULATED INTERNAL ENERGIES FOR POLYCHLORINATED-BIPHENYLS, Journal of the American Society for Mass Spectrometry, 5(1), 1994, pp. 44-52
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.