Electron Affinities of Some Halogen Molecules and the Charge-Transfer Frequency

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Abstract
In an attempt to fix the value of the vertical electron affinity of the iodine molecule EI2v the potential curve for I2— has been constructed from semiempirical considerations. From this curve the value of EI2v is estimated to be 1.7±0.5 eV, in apparent contradiction to the value around 0 eV indicated by the electron‐capture experiments. It is suggested that the electron‐capture experiment may measure the vertical energy for capture to give an excited state of the I2— ion, thus explaining the discrepancy. Having fixed the value for EI2v, the relationship predicting the charge transfer frequency for iodine complexes is reexamined critically. By fitting curves to the data for three different types of complexes (π donors with I2, amine donors with I2, and complexes with I atoms), three sets of empirical constants required to force the fit are determined. An attempt is then made to calculate these sets of constants from a priori considerations. The success of this treatment supports the general theory, indicating especially that no major terms have been omitted from the estimation of the energy difference, W1—W0. Furthermore, the general argument supports the value of EI2v obtained in the earlier part of the paper as well as the reliability of the estimation of the other parameters. The analysis supports the perpendicular model for the geometry of the complexes between I2 and π donors. Finally, similar analyses are presented for other halogens; Br2, Cl2, and ICl. Estimates of the vertical electron affinity for these halogens from the empirical potential curves agree reasonably well with estimates from the charge transfer frequency, in support of the results for I2.

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