Weak association between alkali metal ions and mononegative ions of aromatic hydrocarbons

Extra hyperfine splittings due to alkali metalal cations have been observed in the electron spin resonance (E.S.R.) spectra of biphenyl, naphthalene, anthracene and pyrene mononegative ions. It is interpreted as being due to ion-pair association between the aromatic anions and the metal cations. The unpaired spin densities at the metal nuclei are estimated using the data from the atomic beam experiments. In the case of biphenyl-alkali metal systems the appropriateness of the model described in a previous paper has been checked by studying the temperature dependence of the cation hyperfine splittings and the E.S.R. spectra of some methyl substituted biphenyl anions. The structure of the ion-pairs are discussed using Mulliken's theory of charge-transfer complexes.