An electron transmission technique has been employed to determine the positions of the three π‐negative ion states (configurations π12π22π23π41, π21π22π23π15, and π21π22π23π16) 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.

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