Ab Initio Calculations for Hydrocarbons:  Enthalpy of Formation, Transition State Geometry, and Activation Energy for Radical Reactions

Abstract

A quantum chemical investigation is presented for the determination of accurate kinetic and thermodynamic parameters for hydrocarbon radical reactions. First, standard enthalpies of formation are calculated at different levels of theory for a training set of 58 hydrocarbon molecules, ranging from C 1 to C 10 , for which experimental data are available. It is found that the CBS-QB3 method succeeds in predicting standard enthalpies of formation with a mean absolute deviation of 2.5 kJ/mol, after a systematic correction of -1.29 kJ/mol per carbon atom and -0.28 kJ/mol per hydrogen atom. Even after a systematic correction, B3LYP density functional theory calculations are not able to reach this accuracy, with mean absolute deviations of 9.2 (B3LYP/6-31G(d)) and 12.9 kJ/mol (B3LYP/6-311G(d,p)), and with increasing deviations for larger hydrocarbons. Second, highlevel transition state geometries are determined for 9 carbon-centered radical additions and 6 hydrogen additions to alkenes and alkynes and 10 hydrogen abstraction reactions using the IRCMax(CBS-QB3//B3LYP/6-311G-(d,p)) method. For carbon-centered radical addition reactions, B3LYP/6-311G(d,p) slightly overestimates the length of the forming C-C bond as compared to the IRCMax data. A correlation to improve the agreement is proposed. For hydrogen addition reactions, MPW1K density functional theory (MPW1K/6-31G(d)) is able to locate transition states. However, the lengths of the forming C-H bonds are systematically longer than reference IRCMax data. Here, too, a correlation is proposed to improve the agreement. Transition state geometries for hydrogen abstraction reactions obtained with B3LYP/6-311G(d,p) show good agreement with the IRCMax reference data. Third, the improved transition state geometries are used to calculate activation energies at the CBS-QB3 level. Comparison between both CBS-QB3 and B3LYP density functional theory predictions shows deviations up to 25 kJ/mol. Although main trends are captured by B3LYP DFT, secondary trends due to radical nucleophilic effects are not reproduced accurately.

Enthalpies of Formation and Bond Dissociation Energies of Lower Alkyl Hydroperoxides and Related Hydroperoxy and Alkoxy Radicals

“…The final calculated enthalpies of formation and their associated uncertainties are relatively insensitive to our assumed uncertainty in the computed reaction enthalpies. As a check on the validity of the results, atomization calculations have also been performed at the same levels of theory; the systematic discrepancies noted by Saeys et al 16 for CBS-QB3 have been borne in mind, however.…”

Gas phase enthalpies of formation for aminonitroacetylene, aminonitromethane, and diaminodinitromethane: A Gaussian-4 (G4) theoretical study

“…In the current work, we employ the Gaussian-4 (G4) [5] composite method level of theory within Gaussian 09 (G09) [6] and apply the atomization energy approaches in ref. [7] and ref. [8,9] to provide additional ∆ f H • (g) estimates for these compounds.…”

“…284.0 -66.4 -84.0 current work a atomization energy approach as described in ref. [7]. b atomization energy approach as described in ref.…”

Hydrogen Radical Additions to Unsaturated Hydrocarbons and the Reverse β‐Scission Reactions: Modeling of Activation Energies and Pre‐Exponential Factors

“…It is well known that DFT methods, and in particularly the B3LYP functional [40,41] which is used for the geometry optimization in the CBS-QB3 compound method, pose difficulties to determine accurate transition state geometries for hydrogen addition reactions. Therefore, the transition state geometry is determined as described previously by Saeys [54] . First, the transition state is optimized at the MPW1K/6-31G(d) level using standard transition state search algorithms provided by Gaussian 03.…”

“…Rate coefficients are calculated according to the methodology described by Saeys et al, [54] based on the CBS-QB3 method of Montgomery et al [52] . It is well known that DFT methods, and in particularly the B3LYP functional [40,41] which is used for the geometry optimization in the CBS-QB3 compound method, pose difficulties to determine accurate transition state geometries for hydrogen addition reactions.…”