Transitions intramolecular electron transfer

The interconversion between different spin states is closely related to the intersystem crossing process in excited states of transition-metal complexes. Hence, much of the interest in the rates of spin-state transitions arises from their relevance to a better understanding of intersystem crossing phenomena. The spin-state change can alternatively be described as an intramolecular electron transfer reaction [34], Therefore, rates of spin-state transitions may be employed to assess the effect of spin multiplicity changes on electron transfer rates. These aspects have been covered in some detail elsewhere [30]. 

T. J. Kemp, University of Warwick Noting the very low quantum yield for intramolecular electron transfer in low temperatures displayed by your porphyrin-quinone model compound, would it not be possible to shock-freeze a solution undergoing irradiation at a higher temperature (and giving a workable concentration of paramagnetic species) in order to determine a low-temperature spectrum with the particular aim of observing a possible Am = 2 transition ... 

Pressure provokes transition of the linear (extended) conformation into the bent (V-like) one. (The V-like form is more compact and occupies a smaller volume.) It is obvious that the V-like form is favorable in respect of intramolecular electron transfer from the donor (the aniline part) to the acceptor (the pyrene part). In the utmost level of the phenomenon, the donor part transforms into the cation-radical moiety, whereas the acceptor part passes into the anion-radical moiety. Such transformation is impossible in the case of the extended conformation because of the large distance between the donor and acceptor moieties. The spectral changes observed reflect this conformational transition at elevated pressures. 

Trifluoroethanol (TFE, CE3CH2OH) also demonstrates high H-bond activity. The dyad system in which a radical and electron-donor parts are linked directly undergoes intramolecular electron transfer on substitution of TEE for toluene as a solvent. The transition was interpreted as a marked effect of hydrogen bonding (or reversible protonation) of the anionic R-0 structure with TFE (Nishida et al. 2005). Scheme 5.17 depicts this transition. 

Further work by Anson s group sought to find the effects that would cause the four-electron reaction to occur as the primary process. Studies with ruthenated complexes [[98], and references therein], (23), demonstrated that 7T back-bonding interactions are more important than intramolecular electron transfer in causing cobalt porphyrins to promote the four-electron process over the two-electron reaction. Ruthenated complexes result in the formation of water as the product of the primary catalytic process. Attempts to simulate this behavior without the use of transition-metal substituents (e.g. ruthenated moieties) to enhance the transfer of electron density from the meso position to the porphyrin ring [99] met with limited success. Also, the use of jO-hydroxy substituents produced small positive shifts in the potential at which catalysis occurs. 

In principle, a great deal of information concerning intramolecular electron transfer is available from IT absorption band measurements. Optical electron transfer is rapid on the vibrational time-scale and, as illustrated in Figure 7, the optical transition is a vertical process in the Franck— Condon sense. 

A second way to overcome this spin conservation obstacle is via reaction of 302 with a paramagnetic (transition) metal ion, affording a superoxometal complex (Fig. 4.1, Reaction (3)). Subsequent inter- or intramolecular electron-transfer processes can lead to the formation of a variety of metal-oxygen species (Fig. 4.2) which may play a role in the oxidation of organic substrates. 

The MO 34 iit the eight-orbital model is predominantly the AO Xs- An electronic transition from an MO occupied in the ground state (3i, (f>2, or 3s) to the MO 34 is in effect a transition from an MO to essentially the localized AO Xs-Thus, we may regard such a transition as an intramolecular electron transfer. 

This procedure of including several AOs on an atom in the linear combination of atomic orbitals (LCAO) scheme and obtaining MOs which are essentially pure AOs may prove useful in future work for studying intramolecular electron-transfer processes and n 7t transitions. 

An explanation can be found in terms of the much lower reduction potential of the indole-type radical cations as compared to the phenylalkanoic acid ones, which results in a greater stabilization of the positive charge on the aromatic system thus opposing intramolecular electron transfer. In other words, a later transition state is expected in the decarboxylation of indole-type radical zwitterions as compared to the phenylalkanoic acid ones, with an increased importance of the stability of the carbon centered radical. 

Kuznetsov, A.M. and Ulstrup, J. (1981) Long-range intramolecular electron transfer in aromatic radical-anions and liinuclear transition-metal complexes. Journal of Chemical Physics, 75, 2047-2055. 

The examples of electronic transitions in transition metal complexes discussed here have emphasized solvatochromic effects and often involve intramolecular electron transfer. The role of the solvent in electron transfer is an important aspect of these processes and is discussed in more detail in chapter 7. Many electronic transitions do not display the type of solvent effects considered here but depend instead on bulk solvent properties. The systems discussed above interact in a specific way with the solvent usually as a result of a significant change in the dipole moment of the molecule or the complex ion as a result of the electronic transition. There are many other interesting aspects of this area of spectroscopy which have been discussed in more detailed treatments [38, 43]. 

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