Hiickel calculations allyl system

Figure 14 Basis set orbitals for the allyl system in Hiickel calculations...

The studies of such closely related structures also ruled out any possible steric effects and the driving influence for reaction rate enhancement has to be seen in the oxygen atom in the y-allylic position (C6 of the Claisen system. Scheme 14). In previous reports [23], Carpenter and Burrows had developed a model to predict the influence of substituents on various pericyclic reactions based on Hiickel orbital energy calculations. According to this approach, a n-... 

Because p and d AOs have nodes at the nuclei, Ps(X) can be nonzero only if the SOMO that contains the unpaired a electron has some s character. However, as discussed in the section on UHF calculations, spin polarization in subjacent MOs leads to nonzero local a and P spin density in a MOs, so even when the unpaired electron occupies an MO with no s character, hfcs can still be observed. The clearest manifestation of this phenomenon is the observation of hydrogen hfcs in planar 7t systems, such as the allyl and benzyl radicals, discussed earlier. One might conclude that only methods that account for CT spin polarization by Jt electrons are capable of yielding useful predictions of ESR spectra. However, the observation by McConnell that hydrogen hfcs in planar hydrocarbons are usually proportional to the 7t spin populations on the carbons to which they are attached makes it possible to arrive at quite reasonable estimates of relative ax based on simple Hiickel- or PPP-type calculations on just the Jt electrons. 

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