Structure-dependence in Initial Decomposition of
trans-1,2-Dimethylcyclohexyl Isomers: Kinetic Exploration and
Conformational Analysis
Abstract
Cyclohexyl radicals are crucial primary intermediates in combustion of
fossil and alternative fuels. They would present the inherent
conformation feature, i.e. diverse conformers retained in
inversion-topomerization pathways, jointly controlled by the varying
radical site and specific spatial positions of alkyl side chains on
“easy-distortion” cyclic ring. These conformers for one radical have
different energies and thermodynamics, and are highly expected to
influence their subsequent decomposition reactions in terms of
energetics and kinetics. To reveal such impact, all conformational
structures and their interconversion mechanisms for
trans-1,2-dimethylcyclohexyl isomers were explored by employing quantum
chemical calculations coupled with transition state theory. Originated
from distinct conformers, all accessible transition states were
explicitly identified in different reaction paths for each type of
intramolecular H-transfer or β-scission, and then were carefully used in
computing rate coefficients. The kinetic predictions demonstrate that
the fairly speedy equilibrium among conformers would be established for
one isomer via conformation before they proceed the initial
decomposition over 300-2500 K. This allows thoroughly evaluating the
contribution of various conformers to the kinetics for multiple paths in
one reaction regarding to their thermodynamic properties. Moreover,
conformational analysis elucidates that H-transfers exhibit strong
structure dependence. Note that the most favorable 1,5 H-transfer is
only feasible for one twist-boat with radical site in axial side chain
accompanied by one isoclinal methyl group. The results for β-scissions
are affected by steric energies and substituent effects remained in
conformational structures. These findings facilitate to finally suggest
the proper kinetic parameters for each decomposition reaction with the
aim of their potential implication in kinetic modelling.