Abstract
The kinetic and electron density flows are studied theoretically for the gas phase pyrolysis of cubane via 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(k/s−1)1barMN15-L = (15.63) − (48.99 kcal mol−1)/RT 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: η-1-13-C†C†FFFC†C†FFFC†C†-2-6-[C]2C†[F]2[C†]2C†-0.
Highlights
The fascination of the structure and unusual physicochemical properties of the non-natural compounds have motivated scientists to focus on impossible compounds – those that break the classical rules of chemistry
The cage opening of cubane (A / B) is exergonic ranging from À259 to À305 kJ molÀ1 and exothermic ranging from À225 to À272 kJ molÀ1 depending on the employed computational method
A comparison between the available experimental outcomes with the results of CBS-QB3 and MN15-L/maug-cc-pVTZ level of theories indicates that the former level can accurately predict energetic results
Summary
Decomposition products of cubane and methylcubane were studied by Li et al. in a micro- ow tube reactor using collision induced dissociation mass spectrometry from room temperature to 1000 K They identi ed a set of pyrolysis products and found that the effect of methyl functional group on the stability of cubane is insigni cant. They studied the electronic structure and energetic stability of species using the quantum chemical calculations. Shyamala et al. studied various decomposition pathways of cubane by means of quantum chemical calculations; they proposed various decomposition pathways and subsequent reactions based on the previous experimental ndings They reported heat of formation for all species; activation enthalpies and activation Gibbs free energies for all studied channels. The ELF localization domains for all the stationary points and turning points connecting the successive structural stability domains (SSD) were visualized using the UCFC Chimera program.
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