Through-bond electron exchange

Nelsen and coworkers determined a barrier to inversion through the planar form in 2 and 3 to be approximately 2 kcalmol-1 by variable temperature ESR spectroscopy [59]. Gerson and coworkers found, also by ESR spectroscopy, that the frequency of electron exchange between the two sites in 4, which is equivalent to rotation about the central bond, can vary between < 106 and > 109 s-1 depending the degree of steric hindrance to planarity [60]. Recent calculations also provide very small barriers to inversion through the planar form [56,57]. It is apparent, therefore, that for most synthetic purposes most alkene radical cations can be considered as essentially planar with effective delocalization over the two sp2-hybridized C atoms, and they will be considered as such in this chapter. 

Unpaired electronic density can be delocalized onto the various nuclei of the complex via through-bond scalar hyperfine interactions involving occupied orbitals containing s-character (direct interaction or polarization according to the Fermi mechanism, Wertz and Bolton (1986)). Random electron relaxation thus produces a flip-flop mechanism which affects the nuclear spin and increases nuclear relaxation processes (Bertini and Luchinat, 1996). Since these interactions are isotropic, they do not depend on molecular tumbling and re is the only relevant correlation time for non-exchanging semi-rigid complexes. Moreover, only electronic spin can be delocalized via hyperfine interactions (no orbital contribution) and the contact re-... 

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