Impact of conformational structures on primary decomposition of cis-1,2-dimethylcyclohexyl isomers: A theoretical study

Abstract

The different orientations of two methyl groups in “strain-free” cyclic structure generate multiple conformational structures for dimethyl-substituted cyclohexanes. These conformational structures are most likely to affect the radical stabilities, activation energies, and rate coefficients of key types of reactions in dimethyl-substituted cyclohexane combustion. The conformational inversion-topomerization mechanism among various conformers for cis-1,2-dimethylcyclohexyl isomers has been explored by applying high-level quantum electronic-structure methods and transition state theory (TST). Intramolecular H-transfers and β-scissions were also investigated to fundamentally unravel the way how the conformational structures impact their initial decomposition. The present kinetic predictions show that conformational changes are much more rapid compared with the primary decomposition of cis-1,2-dimethylcyclohexyl isomers. It contributes to the establishment of quasi-equilibrium condition for various conformers retained in each radical and ensures the coexistence of all conformers over 300–2500 K. For the primary decomposition, the intramolecular H-transfers are greatly influenced by the conformational structures. Of particular interest is to observe that 1,4 and 1,5 H-transfers that shift the radical site between side chain and ring are only feasible for chair and twist-boat conformers with the radical site locating in axial side chain. Additionally, the β-scissions of cis-1,2-dimethylcyclohexyl isomers also exhibit the dependence on the conformational structures in aspect of steric energy and substituent effect. Furthermore, facilitated by the speedy equilibration among distinct conformers for each isomer, the contribution of each conformer to kinetic predictions for the initial decomposition was systemically evaluated in terms of the temperature-dependent population for diverse conformers obtained by Boltzmann distribution, and then the appropriate rate parameters for each decomposition type were finally recommended.