Low-energy electron interactions with organic molecules: Negative ion states of fluorobenzenes

An electron transmission technique has been employed to determine the positions of the three π-negative ion states (configurations π₁²π₂²π₃ ²π₄¹, π₁²π ₂²π₃²π₅ ¹, and π₁²π₂ ²π₃²π₆¹) of benzene and fluorobenzenes in the vapor phase. These are 1.13 (1.35), 1.13 (1.35), (4.80) eV for benzene; 0.82 (0.91), (1.40), (4.66) eV for fluorobenzene; 0.53 (0.62), (1.41), (4.51) eV for p-difluorobenzene; (0.77), (0.77), (4.48) eV for 1,3,5-trifluorobenzene; 0.34, (0.50), (1.29), (4.51) eV for 2,3,5,6- tetrafluorobenzene; <0.15 (0.36), (1.19), (4.53) eV for pentafluorobenzene; and (0.42), (0.42), (4.50) eV for hexafluorobenzene. The numbers in parentheses are the vertical attachment energies, and those not in parentheses are the 0→0 transitions. On the basis of these data, the first π-electron affinities (E.A.) of the isolated molecules of these compounds are equal to -1.13, -0.82, -0.53, > -0.77, -0.34, > -0.15, > -0.42 eV for benzene, fluorobenzene, difluorobenzene, 1,3,5-trifluorobenzene, 2,3,5,6- tetrafluorobenzene, pentafluorobenzene, and hexafluorobenzene, respectively. The present results, therefore, suggest that the π-electron affinity of C6F6 is >0.0 eV, although C₆F₆ is known to have a positive (+1.8 eV) E.A. and although the parent ion, C₆F₆ ^-, is known to form with a very large electron attachment cross section at ∼0.0 eV and to be long lived (∼12 μsec). These findings are reconciled, discussed in connection with previously published data, and are theoretically treated. They extend our understanding of the negative ion states of substituted benzenes.