Thermochemical and Kinetic Analysis on the Reactions of Neopentyl and Hydroperoxy-Neopentyl Radicals with Oxygen:  Part I. OH and Initial Stable HC Product Formation

Abstract

Thermochemical properties for reactants, intermediates, products, and transition states in the neopentyl radical + O2 reaction system are analyzed with ab initio and density functional calculations to evaluate reaction paths and kinetics for neopentyl oxidation. Enthalpies of formation (Δ H f ° 298) are determined using isodesmic reaction analysis at the CBS-Q composite and density functional levels. The entropies (S°298) and heat capacities C p (T) (0 ≤ T/K ≤ 1500) from vibrational, translational, and external rotational contributions are calculated using statistical mechanics based on the vibrational frequencies and structures obtained from the density functional study. Potential barriers for the internal rotations are calculated at the B3LYP/6-31G(d,p) level, and hindered rotational contributions to S°298 and C p (T)'s are calculated by using direct integration over energy levels of the internal rotation potentials. The kinetic analysis on reactions of neopentyl with O2 is performed using enthalpies at the CBS-Q calculation level. The reaction forms a chemically activated neopentyl peroxy adduct with an energy of 38.13 kcal mol-1. The energized adduct can be stabilized, dissociate back to reactants, or isomerize to the hydroperoxy-neopentyl radical. The isomer can dissociate to 3,3-dimethyloxetane + OH, to isobutene + CH2O + OH, to methyl + 2-methyl-2-propenyl-hydroperoxide, isomerize back to the neopentyl peroxy radical, or further react with O2. The Δ H f ° 298 values for the neopentyl, neopentyl peroxy, and hydroperoxy-neopentyl radicals are calculated to be 10.52, −27.61, and −9.43 kcal mol-1, respectively, at the CBS-Q level. Rate constants to products and stabilized adducts (isomers) of the chemically activated neopentyl peroxy are calculated as functions of pressure and temperature using quantum Rice−Ramsperger−Kassel (QRRK) analysis for k(E) and a master equation analysis for the pressure falloff. An elementary reaction mechanism is constructed to model the experimental OH formation profile; the concentrations of initial products 3,3-dimethyloxetane and isobutene are also calculated by the model and compared with the experimental results. Kinetic parameters for intermediate and product-formation channels of the neopentyl + O2 system are presented versus temperature and pressure.

Modeling and experimental investigation of methylcyclohexane ignition in a rapid compression machine

“…The two sets of rates based on experiments with cyclohexane provide enough chain branching through 6-membered transition state rings to reproduce the observed NTC character of the ignition. Electronic structure and statistical mechanics calculations [33] are needed to better explain the differences in rate constants between n-/iso-alkanes and cyclo-alkanes.…”

Enthalpy of formation and bond energies on unsaturated oxygenated hydrocarbons using G3MP2B3 calculation methods

“…The standard deviation between the two methods, from several work reactions, is ±1.12 for CH 3 C( O)OOH, ±0.12 for C(OOH)H 2 CHO, ±0.24 for C·H 2 C( O)OOH, ±0.74 for CH 3 C( O)OO·, ±0.48 for C(OOH)H 2 C·O, and ±0.17 for C(OO·)H 2 CHO. A recent study by Sun and Bozzelli [16] compares results for enthalpy calculations on neopentyl radical, neopentaldehyde, and hydroperoxy-neopentyl radicals among the B3LYP/6-31G(d,p), B3LYP/6-311++G(3df,2p), and CBS-Q//B3LYP/6-31G(d,p) methods. The standard deviation between B3LYP/6-31G(d,p) and CBS-Q//B3LYP/6-31G(d,p) on six species varies from ±0.7 to ±1.6 kcal mol −1 and the standard deviation is ±0.38 to ±1.76 kcal mol −1 between B3LYP/6-311++G(3df,2p) and CBS-Q//B3LYP/6-31G(d,p [12] with DFT and −39.52 [19] by CBSq//MP2 (full)/6-31G(d).…”

Detailed chemical kinetic oxidation mechanism for a biodiesel surrogate

“…Reactions of hydroperoxy alkyl and hydroperoxy alkyl-ester radicals (QOOH) are displayed in Figure 6, including the second addition of O 2 forming hydroperoxy peroxy radicals (O 2 QOOH), the decomposition to cyclic ethers plus OH, and the C-O -scission decomposition to HO 2 and alkyl or alkyl-ester radicals. Other reactions of C-C -scissions have not been taken in account because of their higher activation energy [ 28].…”

“…The rate expression of the last reaction type is written as the reverse addition of olefin + HO 2 , with different activation energies depending on whether the HO 2 adds to a primary or secondary carbon atom. The direct eliminations from RO 2 (leading to olefins + HO 2 ) which intervenes in the more recent scheme proposed for the low temperature of alkyl radicals [28][29][30] were not included in the methyl decanoate mechanism and were also not included in the nheptane and iso-octane mechanisms on which it is based. When developing these mechanisms it was believed that the channel of formation of olefins plus HO 2 occurred via C-C -scissions following isomerizations through 5 member ring and kinetic parameters of these and related reactions were calibrated to reproduce the formation of olefins.…”

“…Reactions involved in the low temperature region have more uncertainties: this is the case for the reactions of isomerization of peroxy radicals (RO 2 ) to hydroperoxy radicals (QOOH). Rate constants for these reactions are from Curran et al [ 21] This treatment does not take into account direct eliminations from RO 2 (leading to olefins + HO 2 ) which intervenes in the more recent scheme proposed for the low temperature of alkyl radicals [28][29][30]. Isomerizations of RO 2 to QOOH through cyclic transition states involving the ester function are very uncertain (as shown in Figure 13).…”