Unravelling the kinetics and molecular mechanism of the degenerate Cope rearrangement of bullvalene
The kinetics and molecular mechanism of the gas phase degenerate Cope rearrangement (DCR) of bullvalene have been investigated by applying the quantum mechanical calculations. Highly accurate energies (CBS-QB3 and CBS-APNO) and RRKM calculations were employed to study the kinetics and ‘fall-off’ behavior. It was found that the DCR of bullvalene (C3v) occurs through a bishomoaromatic transition structure (C2v) with an energy barrier of ~49 kJ.mol^-1. The calculated activation energy and enthalpy were in good agreement with the available values in literatures, but lower than that of common Cope rearrangement; this result is related to the high stabilization energy due to the interaction of the allyl fragments in the bishomoaromatic transition structure. Fall-off curve revealed that the TST breaks down slightly in estimating the high-pressure limit of reaction rate and also obtaining the reaction as bimolecular is experimentally impossible. The synergic effect of ELF, NCI, and QTAIM has been used to study the molecular mechanism of the DCR of bullvalene at the B3LYP/6-311G(d,p) level of theory. The catastrophe sequences for DCR of bullvalene was 5-C†[F]2TS[F†]2C-0 which include the following four steps: i) homolytic rupture of C1-C7 bond and formation of two pseudoradical centers on C1 and C7 atoms; ii) destruction of the pseudoradical centers; iii) formation of new pseudoradical centers on C3 and C5 atoms; and iv) C- to -C coupling of the pseudoradical centers and formation of new C3-C5 bond; where the electron density rearrangement takes place in asynchronous manner.