Nuclear Quadrupole Resonance Study of Charge Transfer Complexes: Chloranil Complexes

Abstract 35Cl nuclear quadrupole resonance frequencies were measured in electron donor-acceptor complexes between chloranil (tetrachloro-p-benzoquinone) and such donors as aromatic hydrocarbons, polyaromatics and amines. Some correlations between the resonance frequencies and infrared spectra were found and discussed in a qualitative way in terms of the π-electron transfer between donors and the chloranil molecule as acceptor. A Zeeman effect experiment was carried out on a single crystal of the 1:2 chloranil-8-hydroxyquinoline complex. The direction of one of the two non-equivalent C-Cl bonds obtained slightly differs from that determined by an X-ray diffraction study. Possible effects of the charge transfer on the resonance frequency and the asymmetry parameter are discussed according to the Mulliken theory of ’weak’ charge-transfer complex. No simple relations could be found between the experimental resonance frequencies and the ionization potential of the donor. This suggests that crystal field effects are rather large in comparison with the effect of charge transfer interaction.