Thermochemical Properties for Isooctane and Carbon Radicals: Computational Study

Abstract

Thermochemical properties for isooctane, its internal rotation conformers, and radicals with corresponding bond energies are determined by use of computational chemistry. Enthalpies of formation are determined using isodesmic reactions with B3LYP density function theory and composite CBS-QB3 methods. Application of group additivity with comparison to calculated values is illustrated. Entropy and heat capacities are determined using geometric parameters, internal rotor potentials, and frequencies from B3LYP/6-31G(d,p) calculations for the lowest energy conformer. Internal rotor potentials are determined for the isooctane parent and for the primary, secondary, and tertiary radicals in order to identify isomer energies. Intramolecular interactions are shown to have a significant effect on the enthalpy of formation of the isooctane parent and its radicals. The computed standard enthalpy of formation for the lowest energy conformers of isooctane from this study is -54.40 ± 1.60 kcal mol(-1), which is 0.8 kcal mol(-1) lower than the evaluated experimental value -53.54 ± 0.36 kcal mol(-1). The standard enthalpy of formation for the primary radical for a methyl on the quaternary carbon is -5.00 ± 1.69 kcal mol(-1), for the primary radical on the tertiary carbon is -5.18 ± 1.69 kcal mol(-1), for the secondary isooctane radical is -9.03 ± 1.84 kcal mol(-1), and for the tertiary isooctane radical is -12.30 ± 2.02 kcal mol(-1). Bond energy values for the isooctane radicals are 100.64 ± 1.73, 100.46 ± 1.73, 96.41 ± 1.88 and 93.14 ± 2.05 kcal mol(-1) for C3•CCCC2, C3CCCC2•, C3CC•CC2, and C3CCC•C2, respectively. Entropy and heat capacity values are reported for the lowest energy homologues.

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

“…Since the publication of the latest comprehensive iso-octane model [1], several computational studies have been reported for iso-octane thermochemistry and kinetics. Snitsiriwat and Bozzelli [41] computed the standard heat of formation, entropy, and specific heat capacities of iso-octane and its radicals at the CBS-QB3//B3LYP/6-31G(d,p) level of theory. 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.…”

“…Computational standard heat of formation values of iso-octane [41], iso-octyl radicals [41], and species involved in the potential energy surfaces of secondary [42] and tertiary iso-octyl radical + O 2 [43] are available in the literature. These values, calculated at the CBS-QB3 level of theory, were compared to those calculated using group additivity, as shown in Table 1.…”

“…Comparison of calculated[41][42][43] and THERM estimated heats of formation of different species. Species that exhibit large discrepancies are shown in bold.…”

Thermochemistry, Reaction Paths, and Kinetics on the Secondary Isooctane Radical Reaction with 3 O2

“…The enthalpies of formation of the isooctane parent molecule and each of the isooctane alkyl radicals, as well as BDE for each of the radicals were reported by Snitsiriwat and Bozzelli [10] and are summarized in Table III for comparison. The enthalpies of formation of the secondary isooctane peroxy radicals and the enthalpies of formation of the resulting products from the studied reaction channels are shown in Tables IV and V. The B3LYP/6-31G(d,p) calculations are in good agreement with the CBS-QB3 calculations when isodesmic work reactions are used.…”

“…Enthalpies of Formation at 298 K of the Isooctane Parent and Radicals Reported by Snitsiriwat and Bozzelli[10] …”