Captodative effect ground-state effects

Similarly to the triphenylmethyl system, captodative-substituted 1,5-hexa-dienes, which can be cleaved thermally in solution into the corresponding substituted allyl radicals [15], dissociate more easily than dicaptor-substituted systems (Van Hoecke et al., 1986). Since ground-state and radical substituent effects cannot be separated cleanly, not only because of electronic but also because of steric effects, a conclusive answer cannot be provided. 

Several attempts have been made to analyse the captodative effect through rotational barriers in free radicals. This approach seems to be well suited as it is concerned directly with the radical, i.e. peculiarities associated with bond-breaking processes do not apply. However, in these cases also one has to be aware that any influence of a substituent on the barrier height for rotation is the result of its action in the ground state of the molecule and in the transition structure for rotation. Stabilization as well as destabilization of the two states could be involved. Each case has to be looked at individually and it is clear that this will provide a trend analysis rather than an absolute determination of the magnitude of substituent effects. In this respect the analysis of rotational barriers bears similar drawbacks to all of the other methods. 

The study of the rotational barriers in captodative-substituted radicals leads to the following conclusions the barriers are noticeably lower or higher than in cases of dicaptor or didonor substitution. This can be interpreted as the consequence of a captodative effect in these systems. However, the amount of special influence on the barrier height in energetic terms is small and may sometimes not exceed the numerical uncertainties. A derivation of absolute values for stabilization energies of captodative-substituted radicals by this procedure is not possible, since both ground and transition states are affected by substitution. The lowering of the barriers... 

The quantum chemical studies have not reached a unanimous conclusion. The more sophisticated procedures predict that in some captodative substituted systems an additive or a slightly more than additive substituent effect is possible. The calculations, particularly those of Leroy, have also contributed to the belief that the study of substituent effects requires the consideration of their influence in the ground and final states of the model system. 

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