Measurement of Ionization Potentials from Contact Charge Transfer Spectra

Abstract

THE use of charge transfer spectra for the evaluation of ionization potentials is well known. Thus Foster has found an empirical relationship between the ionization potentials of condensed ring aromatic hydrocarbons and the frequencies of maximum absorption of their charge transfer complexes with chloranil in carbon tetrachloride¹. Foster's formula is limited to the condensed ring aromatic hydrocarbons because the binding of electron donors to acceptors in charge transfer complexes is not caused entirely by charge transfer; intermolecular forces vary between different classes of molecules and influence complex formation. In general, the relationship between the ionization potential and the frequency of maximum absorption of the charge transfer complex is given by where I_p is the ionization potential of the electron donor, E_A is the electron affinity of the electron acceptor, e²/r is the coulombic interaction between the two and Δ represents all the other intermolecular forces². There exists a type of charge transfer between molecules which does not produce a bound complex and is termed contact charge transfer³. The electron donors and acceptors are randomly disposed in solution but when they come into contact with each other momentary charge transfer takes place which gives rise to an absorption spectrum. One electron acceptor which takes part in contact charge transfer is oxygen⁴. Measurements made of its equilibrium constant with electron donors yield a value of zero^4,5. Furthermore, the absorption band caused by this type of charge transfer increases to higher frequency without having a maximum absorption peak, the type of behaviour that can be predicted for transitions occurring between energy levels which show no minima, that is two non-bonding levels. Because there is no binding in contact “complexes” it would appear that a very general relationship can be set up between the ionization potentials of electron donors and the onset of absorption of the charge transfer band arising with oxygen.