Thermochemistry, Reaction Paths, and Kinetics on the tert-Isooctane Radical Reaction with O₂

Abstract

Thermochemical properties of tert-isooctane hydroperoxide and its radicals are determined by computational chemistry. Enthalpies are determined using isodesmic reactions with B3LYP density function and CBS QB3 methods. Application of group additivity with comparison to calculated values is illustrated. Entropy and heat capacities are determined using geometric parameters and frequencies from the B3LYP/6-31G(d,p) calculations for the lowest energy conformer. Internal rotor potentials are determined for the tert-isooctane hydroperoxide and its radicals in order to identify isomer energies. Recommended values derived from the most stable conformers of tert-isooctane hydroperoxide of are -77.85 ± 0.44 kcal mol(-1). Isooctane is a highly branched molecule, and its structure has a significant effect on its thermochemistry and reaction barriers. Intramolecular interactions are shown to have a significant effect on the enthalpy of the isooctane parent and its radicals on peroxy/peroxide systems, the R• + O2 well depths and unimolecular reaction barriers. Bond dissociation energies and well depths, for tert-isooctane hydroperoxide → R• + O2 are 33.5 kcal mol(-1) compared to values of ∼38 to 40 kcal mol(-1) for the smaller tert-butyl-O2 → R• + O2. Transition states and kinetic parameters for intramolecular hydrogen atom transfer and molecular elimination channels are characterized to evaluate reaction paths and kinetics. Kinetic parameters are determined versus pressure and temperature for the chemically activated formation and unimolecular dissociation of the peroxide adducts. Multifrequency quantum RRK (QRRK) analysis is used for k(E) with master equation analysis for falloff. The major reaction paths at 1000 K are formation of isooctane plus HO2 followed by cyclic ether plus OH. Stabilization of the tert-isooctane hydroperoxy radical becomes important at lower temperatures.

A comprehensive iso-octane combustion model with improved thermochemistry and chemical kinetics

“…Later, Auzmendi-Murua and Bozzelli [42] performed energy calculations on the potential energy surface of the secondary iso-octyl radical + O 2 system at the CBS-QB3//B3LYP/6-31G(d,p) level of theory, and provided corresponding thermochemical and kinetic data. Recently, the thermochemistry of species and kinetics of reactions lying on the potential energy surface of tertiary iso-octyl + O 2 system were computationally studied at the CBS-QB3//B3LYP/6-31G(d,p) level of theory [43]. Finally, the pressure-dependent kinetics of iso-octane unimolecular decomposition, radical β-scission, and radical isomerization were computed using the CASPT2-(2e,2o)/6-31+G(d,p)//CAS(2e,2o)/6-31+G(d,p) level of theory and Rice−Ramsperger−Kassel−Marcus/Master Equation (RRKM/ME) analysis [44].…”

A theoretical study of cyclic ether formation reactions

“…In recent years, there has been a proliferation of systematic theoretical studies concerning reaction pathways of importance in the low-temperature oxidation of alkanes [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] . The rapid increase in the number of studies of this kind is due, in part, to more readily available computational resources.…”

Reaction kinetics and thermochemistry of the chemically activated and stabilized primary ethyl radical of methyl ethyl sulfide, CH3SCH2CH2•, with O2 to CH3SCH2CH2OO•

“…Temperature-and pressure-dependent rate constants are calculated using the multichannel, multifrequency quantum Rice-Ramsperger-Kassel (QRRK) analysis for k(E) based on a master equation analysis for falloff and stabilization as implemented in the CHEMASTER code. 34,51 Reduced sets of three vibrational frequencies and their degeneracy plus energy levels of one external rotor are used to yield the ratio of density of states to partition the coefficient, (E)/Q for each adduct (isomer in the chemical activation or dissociation reaction system). The master equation analysis used an exponential-down model for the energy transfer function with (ΔE down o ) 0.9 kcal mol −1 for N 2 as the third body.…”

“…50 In 2014, Snitsiriwat and Bozzelli reported their work related to thermochemistry, reaction paths, and kinetics on the tert-isooctane radical reaction with O 2 . 51 The elementary reaction steps investigated in this paper belong to the following categories 15,34,50,51,55,56 and are illustrated in Figure 1.…”

“…The VTST plot of CH 3 SCH 2 CH 2 -OO• cleavage results in the bond dissociation energy (BDE) of is 33.8 kcal mol −1 , 34,50,51 For Snitsiriwat's tert-isooctane hydroperoxy C-OO• bond dissociation, the transition state occurs at 3.0Å under T = 300 K. 51 The normalized energy versus bond length plot in Figure 3 indicates…”