Transition state, charge separation complex

There is evidence, both experimental and theoretical, that there are intermediates in at least some Sn2 reactions in the gas phase, in charge type I reactions, where a negative ion nucleophile attacks a neutral substrate. Two energy minima, one before and one after the transition state, appear in the reaction coordinate (Fig. 10.1). The energy surface for the Sn2 Menshutkin reaction (p. 499) has been examined and it was shown that charge separation was promoted by the solvent.An ab initio study of the Sn2 reaction at primary and secondary carbon centers has looked at the energy barrier (at the transition state) to the reaction. These minima correspond to unsymmetrical ion-dipole complexes. Theoretical calculations also show such minima in certain solvents, (e.g., DMF), but not in water. "... 

It is clear that these bromodemetallations, proceeding by the SE mechanism in the non-polar solvent chlorobenzene, are extremely complex and that a variety of transition states (e.g. (XII), (XIII), and (XV)) are possible. The main function of the more complicated transition states seems to be that of providing a route for bromodemetallation such that collapse of the transition state yields uncharged molecules and not ionic species. In addition, the transition states also serve to distribute any charge separation over a molecular aggregate. 

Abstract The photochemical properties of transition metal complexes, such as those of iridium(III) or ruthenium(II), can be exploited in various ways to generate charge-separated (CS) states, in relation to the mimicry of the natural photosynthetic reaction centres, or to set multicomponent compounds or assemblies in motion. The first part of the present chapter summarizes the work carried out in our groups (Bologna and Strasbourg) in recent years with iridium(III)-terpy complexes (terpy 2,2,6,6"-terpyridine). The synthesis of multicomponent iridium(III) complexes in reasonable yields has been... 

Many multicomponent transition metal complexes have been elaborated in the course of the last 15 to 20 years, with the aim of inducing charge separation under the action of light, so as to generate reasonably long-lived charge separated (CS) states. The first part of this review article relates to this active field of research. 

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