INTRAMOLECULAR COMPETITION OF PHENYLIC AND BENZYLIC CX BOND BREAKING IN DISSOCIATIVE ELECTRON-ATTACHMENT TO DIHALOTOLUENES

Relative yields of Cl- and Br- (X(-)) from dissociative electron attac hment to o-, m-, and p-ClC6H4CH2Br and BrC6H4CH2Cl have been measured as a function of the incident electron energy in the gas phase. A Cl- or Br- signal was observed for all compounds at incident electron ener gies of 0-1 eV, corresponding to electron attachment to pi orbitals f ollowed by dissociation. Bond breaking is highly selective; the yield of the benzylic X(-) is always substantially higher than the yield of the phenylic X(-) at incident energies below 1 eV. The selectivity is explained by the weaker benzylic CX bond and by symmetry restrictions in pi/sigma* orbital coupling, making the dissociative intramolecular electron transfer from the benzene pi MO into the benzylic CX bond f aster than into the phenylic CX bond. A second (weaker) X(-) band is o bserved in some compounds at incident electron energies around 4.5 eV. The former selectivity of bond breaking is reversed at this incident energy, indicating a different electronic symmetry of the incipient ra dical anion state.