Hyperconjugation coupling

In addition the HF calculations show that no significant hyperconjugative interaction takes place between the a radical center and the p-CH bonds. This view is contradicted by the ESR results of Kawamura and coworkers who conclude from their data that the a-cyclopropyl radical is hyperconjugatively coupled with the p-CH bonds and that the coupling is stronger with the cis bond than with the trans bond. 

Fig. 3. Explanation of the level crossing in Fig. 2 a. a increases from left to right. The A level is stabilized by the decrease in through-space interaction through-bond coupling (hyperconjugation) with the a and a levels of the interposed CH2 destabilizes the S level.

Withdrawal of electrons from orbitals antisymmetric between hydrogen atoms (generally hyperconjugative effects) should lead to a negative change in the coupling constants. ... 

Further support for the rehybridization model was provided by analysis of the spin densities of annelated 1,4-naphthoquinones, n hthalenes, and cyclobutaben-zene radical anions by ESR. The hyperfine coupling constants at the methylene position decrease as ring strain increases. Calculation of spin densities using the rehybridization model leads to an excellent fit of experimental spin densities, while calculation of spin densities using either the Coulson-Crawford hyperconjugation model or INDO do not accurately correlate experimental values. Such results suggest that rehybridization can account for the observed changes in spin densities. 

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