| Abstract
| - Hybrid DFT calculations of the potential energy surface (PES) relative to the O-neophyl rearrangementof a series of ring-substituted 1,1-diarylalkoxyl radicals have been carried out at the UB3LYP/6-31G(d)level of theory. On the basis of the computational data, the rearrangement can be described as a consecutivereaction of the type a ⇆ b → c (see above graphic), and the steady-state approximation could be appliedin all cases to the intermediate b. The first-order rearrangement rate constants [kobs = k1k2/(k-1 + k2)]were thus obtained from the computed activation free-energies and were compared with the experimentalrate constants measured previously in MeCN solution by laser flash photolysis. An excellent agreementis observed along the two series, which strongly supports the hypothesis that the O-neophyl rearrangementof 1,1-diarylalkoxyl radicals proceeds through the formation of the reactive 1-oxaspiro [2,5]octadienylradical intermediate. This is in contrast to previous hypotheses that involve either a long-lived intermediateor the absence of this intermediate along the reaction path. The calculated rearrangement free-energiesdecrease upon going from the methoxy-substituted radical (ΔG° = −16.4 kcal·mol-1) to the nitro-substituted one (ΔG° = −21.8 kcal·mol-1), which follows a trend that is similar to the one observed forthe CAr−O bond dissociation enthalpies (BDEs) of ring-substituted anisoles. This evidence indicates thatin the O-neophyl rearrangement the effect of ring substituents on the strength of the newly formed CAr−Obond plays an important role.
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