Ion-cluster formation and its effect on the electronic spectrum of radical-ion pairs

Abstract

It has previously been shown that the visible-region band in the spectrum of the lithium–benzophenone radical-ion pair is shifted towards the blue by added lithium bromide, and the effect has been interpreted in terms of the formation of multi-ion-pair clusters. It is shown here that a more consistent interpretation of the data can be given in terms of the formation of a well defined 1 : 1 (or possibly 1 : 2) electrostatically bound complex. A general theory is given for interpreting the effect of such complex formation on the band position, and it is deduced that the equilibrium constant for the formation reaction is 580 at room temperature. This value, and the observed magnitude of the spectral shift, agrees with predictions derived from a simple electrostatic treatment of the complex.