Charge-Transfer Forces in Molecular Compounds. II: Benzene-Iodine Complex

It is of much interest to inquire whether the structure of the benzene-iodine complex belongs to the symmetry of C2v or C6v. Here the former is called the R (resting) model and the latter the A (axial) model. Some recent experiments on the infra-red spectra of this complex and the observation of the crystalline state of the benzene-bromine complex support the A model of C6v symmetry. In this article the author concludes from a theoretical consideration that the A model is most probable for the structure of the benzene-iodine complex. In the A model, the charge-transfer takes place from φ0, the MO (molecular orbital) of benzene to φ(σu, 5p), the MO of iodine. The energy loss of the charge-transfer state which is involved in adopting the A model instead of the R model, is supplied by the “disscreening effect” and the interaction energy between the no-bond state and the charge-transfer state. This interaction energy is far larger in the A model than in the R model. The stabilization energy, the dipole moment, the charge-transfer spectrum and the oscillator strength are calculated, and we have obtained fairly good agreement with the observations.