Unraveling the kinetics and molecular mechanism of gas phase pyrolysis of cubane to [8]annulene.

The kinetic and electron density flows are studied theoretically for the gas phase pyrolysis of cubane its cage opening to reach bicyclooctatriene and then thermal rearrangement of bicyclooctatriene to produce [8]annulene which is the experimentally observed major product. The observed kinetic data at the MN15-L/maug-cc-pVTZ level of theory were in good agreement with the experimental results as compared to the CBS-QB3 method. The cage opening and the thermal rearrangement steps at the experimentally employed temperature of 520 K were exergonic and exothermic. The atmospheric rate constants calculated by means of the RRKM theory show that the cage opening is the rate-determining step. The temperature dependence of the rate constant for the cage opening step at the MN15-L level can be expressed as log(/s) = (15.63) - (48.99 kcal mol)/ ln 10. The molecular mechanism of the reactions has been investigated by means of the bonding evolution theory (BET) at the B3LYP/6-311G (d,p) level of theory. The cage opening course is described topologically by cleaving of C1-C2, C4-C8, and C5-C6 single bonds and electron saturation of the C1-C4, C2-C6, and C5-C8 bonds, while the rearrangement of bicyclooctatriene is described by C3-C7 bond rupture, depopulation of C1-C4 and C5-C8 double bonds, and electron saturation of C1-C5, C3-C4, and C7-C8 bonds. Electron density rearrangement along the two successive steps are asynchronous and the sequence of catastrophes can be represented as: -1-13-CCFFFCCFFFCC-2-6-[C]C[F][C]C-0.