Pressure-Dependent Kinetics of Initial Reactions in Iso-octane Pyrolysis

This study focuses on the studies
of the main pressure-dependent
reaction types of iso-octane (iso-C8H18) pyrolysis,
including initial C–C bond fission of iso-octane, isomerization,
and β-scission reactions of the alkyl radicals produced by the
C–C bond fission of iso-octane. For the C–C bond fission
of iso-octane, the minimum energy potentials are calculated at the
CASPT2­(2e,2o)/6-31+G­(d,p)//CAS­(2e,2o)/6-31+G­(d,p) level of theory.
For the isomerization and the β-scission reactions of the alkyl
radicals, the optimization of the geometries and the vibrational frequencies
of the reactants, transition states, and products are performed at
the B3LYP/CBSB7 level, and their single point energies are calculated
by using the composite CBS-QB3 method. Variable reaction coordinate
transition state theory (VRC-TST) is used for the high-pressure limit
rate constant calculation and Rice–Ramsperger–Kassel–Marcus/master
equation (RRKM/ME) is used to calculate the pressure-dependent rate
constants of these channels with pressure varying from 0.01–100
atm. The rate constants obtained in this work are in good agreement
with those available from literatures. We have updated the rate constants
and thermodynamic parameters for species involved in these reactions
into a current chemical kinetic mechanism and also have improved the
concentration profiles of main products such as C3H6 and C4H6 in the shock tube pyrolysis
of iso-octane. The results of this study provide insight into the
pyrolysis of iso-octane and will be helpful in the future development
of branched paraffin kinetic mechanisms.