Potential energy surfaces Cope rearrangements

Even if full potential energy surfaces are not calculated, simple EHT calculations, skilfully coupled with orbital symmetry considerations, can provide insight into complex reactivity problems. This is well exemplified by Hoffmann and Stohrer s analysis of substituent effects on the Cope rearrangement (28). 

Barbaralene [85] undergoes a rapid Cope rearrangement with a doublewell potential. The radical cation was studied using CIDNP by Roth (1987) after one-electron oxidation of [85] by y or X-irradiation. On the time-scale of the CIDNP experiment ( 10 8s), a single-minimum potential energy surface was found, i.e. bishomoaromatic structure [156] was suggested. 

Consideration of the relative entropic contributions to the potential energy surface at the temperatures of the reactions suggest the addition of approximately 9 kcal/mol to the energy of all species described above except 1,5-hexadiene. This then leads to the free energy surface for the reactions including the Cope rearrangement of 1,5-hexadiene described in Scheme 7.97. 

The [1,2] shifts have already been reviewed and will not in general be discussed here. Most of the other ab initio studied have been performed at the SCF level and only few recent studies, viz. the [1,2] and [1,3] shifts in propene and the Cope rearrangement, at the MC-SCF level. The MC-SCF results suggest that SCF theory can be inadequate to describe large parts of these potential energy surfaces and consequently here we discuss in... 

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