On charge transfer spectra

The relationship between the maximum of absorption of a donor‐acceptor complex and the P value of the relevant donor for a series of donors with the same acceptor is reasonable linear. The P value, measure for the energy of the electron in the highest filled orbital, has the advantage of being calculable for many substances of biological interest. The relationship is still linear for low P values and also for complexes with maxima in the near infrared. The relationship is also tested for polar solvents. In case more than one maximum is formed, only one obeys the rule. Sometimes the deviations observed are systematic, e.g. effect of the solvent and are cancelled when the results of two series (two different acceptors) are compared (Foster's rule).