Intervalence band profiles

Does T differ significantly from unity in typical electron transfer reactions It is difficult to get direct evidence for nuclear tunnelling from rate measurements except at very low temperatures in certain systems. Nuclear tunnelling is a consequence of the quantum nature of oscillators involved in the process. For the corresponding optical transfer, it is easy to see this property when one measures the temperature dependence of the intervalence band profile in a dynamically-trapped mixed-valence system. The second moment of the band,... 

The Pt + Pt intervalence transitions of such chain complexes occur in the regions 25,000-18,200 cm 1, 23,600-14,300 cm 1 and 20,600-7,500 cm 1 for chloro-, bromo-, and iodo-bridged complexes, respectively, the trend Cl > Br > I being the reverse of that of the conductivity of the complexes. The transition wavenumbers may be determined either by Kramers-Kronig analysis of specular reflectance measurements or from plots of the excitation profiles of Raman bands enhanced at or near resonance with the Pt I-PtIV intervalence band. The maxima have been found to be related to the Pt —PtIV chain distance, the smaller the latter the less being the intervalence transition energy (3). 

This example relates to a ground-state outer-sphere complex (p) existing in labile equilibrium with reactants A+ and B. Other systems considered include photoelectron emission, directly bonded systems (such as Fe CF+ - Fe Cl +), and charge transfer to and from the solvent. Meyer concentrates on intervalence charge transfer and intramolecular thermal electron transfer in homo-nuclear mixed-valence complexes. He remarks that the ideal experiment will be to measure both IT band energies and the rate of intramolecular electron transfer directly for an ion like [(bipy)2ClRu Hpyz)Ru Cl(bipy)2] , since it will then be possible to sketch the complete energy profile and to obtain the frequency factor for the thermal process. 

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