M-F. Reyniers

First Principles Based Group Additive Values for the Gas Phase Standard Entropy and Heat Capacity of Hydrocarbons and Hydrocarbon Radicals

M. Sabbe, F. De Vleeschouwer, M-F. Reyniers, M. Waroquier, G.B. Marin
Journal of Physical Chemistry A
112 (47), 12235-12251
2008
A1

Abstract 

In this work a complete and consistent set of 95 Benson group additive values (GAVs) for standard entropies S° and heat capacities Cp° of hydrocarbons and hydrocarbon radicals is presented. These GAVs include 46 groups, among which 25 radical groups, which, to the best of our knowledge, have not been reported before. The GAVs have been determined from a set of B3LYP/6-311G(d,p) ideal gas statistical thermodynamics values for 265 species, consistently with previously reported GAVs for standard enthalpies of formation. One-dimensional hindered rotor corrections for all internal rotations are included. The computational methodology has been compared to experimental entropies (298 K) for 39 species, with a mean absolute deviation (MAD) between experiment and calculation of 1.2 J mol−1 K−1, and to 46 experimental heat capacities (298 K) with a resulting MAD = 1.8 J mol−1 K−1. The constructed database allowed evaluation of corrections on S° and Cp° for non-nearest-neighbor effects, which have not been determined previously. The group additive model predicts the S° and Cp° within 5 J mol−1 K−1 of the ab initio values for 11 of the 14 molecules of the test set, corresponding to an acceptable maximal deviation of a factor of 1.6 on the equilibrium coefficient. The obtained GAVs can be applied for the prediction of S° and Cp° for a wide range of hydrocarbons and hydrocarbon radicals. The constructed database also allowed determination of a large set of hydrogen bond increments, which can be useful for the prediction of radical thermochemistry.

Theoretical Study of the Thermodynamics and Kinetics of Hydrogen Abstractions from Hydrocarbons

A.G. Vandeputte, M. Sabbe, M-F. Reyniers, V. Van Speybroeck, M. Waroquier, G.B. Marin
Journal of Physical Chemistry A
111 (46), 11771–11786
2007
A1

Abstract 

Thermochemical and kinetic data were calculated at four cost-effective levels of theory for a set consisting of five hydrogen abstraction reactions between hydrocarbons for which experimental data are available. The selection of a reliable, yet cost-effective method to study this type of reactions for a broad range of applications was done on the basis of comparison with experimental data or with results obtained from computationally demanding high level of theory calculations. For this benchmark study two composite methods (CBS-QB3 and G3B3) and two density functional theory (DFT) methods, MPW1PW91/6-311G(2d,d,p) and BMK/6-311G(2d,d,p), were selected. All four methods succeeded well in describing the thermochemical properties of the five studied hydrogen abstraction reactions. High-level Weizmann-1 (W1) calculations indicated that CBS-QB3 succeeds in predicting the most accurate reaction barrier for the hydrogen abstraction of methane by methyl but tends to underestimate the reaction barriers for reactions where spin contamination is observed in the transition state. Experimental rate coefficients were most accurately predicted with CBS-QB3. Therefore, CBS-QB3 was selected to investigate the influence of both the 1D hindered internal rotor treatment about the forming bond (1D-HR) and tunneling on the rate coefficients for a set of 21 hydrogen abstraction reactions. Three zero curvature tunneling (ZCT) methods were evaluated (Wigner, Skodje & Truhlar, Eckart). As the computationally more demanding centrifugal dominant small curvature semiclassical (CD-SCS) tunneling method did not yield significantly better agreement with experiment compared to the ZCT methods, CD-SCS tunneling contributions were only assessed for the hydrogen abstractions by methyl from methane and ethane. The best agreement with experimental rate coefficients was found when Eckart tunneling and 1D-HR corrections were applied. A mean deviation of a factor 6 on the rate coefficients is found for the complete set of 21 reactions at temperatures ranging from 298 to 1000 K. Tunneling corrections play a critical role in obtaining accurate rate coefficients, especially at lower temperatures, whereas the hindered rotor treatment only improves the agreement with experiment in the high-temperature range.

arbon-Centered Radical Addition and β-Scission Reactions: Modeling of Activation Energies and Pre-exponential Factors

M. Sabbe, A.G. Vandeputte, M-F. Reyniers, V. Van Speybroeck, M. Waroquier, G.B. Marin
Journal of Physical Chemistry A
111 (34), 8416-8428
2007
A1

Abstract 

A consistent set of group additive values ΔGAV° for 46 groups is derived, allowing the calculation of rate coefficients for hydrocarbon radical additions and β-scission reactions. A database of 51 rate coefficients based on CBS-QB3 calculations with corrections for hindered internal rotation was used as training set. The results of this computational method agree well with experimentally observed rate coefficients with a mean factor of deviation of 3, as benchmarked on a set of nine reactions. The temperature dependence on the resulting ΔGAV°s in the broad range of 300–1300 K is limited to ±4.5 kJ mol−1 on activation energies and to ±0.4 on logA (A: pre-exponential factor) for 90 % of the groups. Validation of the ΔGAV°s was performed for a test set of 13 reactions. In the absence of severe steric hindrance and resonance effects in the transition state, the rate coefficients predicted by group additivity are within a factor of 3 of the CBS-QB3 ab initio rate coefficients for more than 90 % of the reactions in the test set. It can thus be expected that in most cases the GA method performs even better than standard DFT calculations for which a deviation factor of 10 is generally considered to be acceptable.

Ab Initio Group Contribution Method for Activation Energies of Hydrogen Abstraction Reactions

M. Saeys, M-F. Reyniers, V. Van Speybroeck, M. Waroquier, G.B. Marin
ChemPhysChem
7 (1), 188-199
2006
A1

Abstract 

The group contribution method for activation energies is applied to hydrogen abstraction reactions. To this end an ab initio database was constructed, which consisted of activation energies calculated with the ab initio CBS-QB3 method for a limited set of well-chosen homologous reactions. CBS-QB3 is shown to predict reaction rate coefficients within a factor of 2–4 and Arrhenius activation energies within 3–5 kJ mol−1of experimental data. Activation energies in the set of homologous reactions vary over 156 kJ mol−1with the structure of the abstracting radical and over 94 kJ mol−1with the structure of the abstracted hydrocarbon. The parameters required for the group contribution method, the so-called standard activation group additivity values, were determined from this database. To test the accuracy of the group contribution method, a large set of 88 additional activation energies were calculated from first principles and compared with the predictions from the group contribution method. It was found that the group contribution method yields accurate activation energies for hydrogen-transfer reactions between hydrogen molecules, alkylic hydrocarbons, and vinylic hydrocarbons, with the largest deviations being less than 6 kJ mol−1. For reactions between allylic and propargylic hydrocarbons, the transition state is believed to be stabilized by resonance effects, thus requiring the introduction of an appropriate correction term to obtain a reliable prediction of the activation energy for this subclass of hydrogen abstraction reactions.

Ab Initio Study of Free-Radical Polymerization: Polyethylene Propagation Kinetics

K. Van Cauter, V. Van Speybroeck, P. Vansteenkiste, M-F. Reyniers, M. Waroquier
ChemPhysChem
7 (1), 131-140
2006
A1

Abstract 

The chain-length dependence of the propagation rate coefficient for the free-radical polymerization of ethylene was investigated on an ab initio basis. Polyethylene was chosen as a test system because of its structural simplicity. Ab initio density functional theory at the B3LYP/6-31g(d) level was applied to study the kinetics of a set of addition reactions of a systematically growing radical alkyl chain to ethylene. These reactions are propagation steps in the free-radical polymerization of ethylene. Special attention was paid to low normal modes corresponding to internal rotations (IR), since the latter are important for an accurate description of the partition functions. The effect of coupling of the IR modes is also discussed. A comparison is made with the propagation rate constant derived from experiment. The results indicate that the propagation rate coefficient has largely converged by the hexyl radical stage, though a weaker chain-length dependence of kpfor longer chains was detected.

Group additive values for the gas phase standard enthalpy of formation of hydrocarbons and hydrocarbon radicals

M. Sabbe, M. Saeys, M-F. Reyniers, G.B. Marin, V. Van Speybroeck, M. Waroquier
Journal of Physical Chemistry A
109 (33), 7466-7480
2005
A1

Abstract 

A complete and consistent set of 95 Benson group additive values (GAV) for the standard enthalpy of formation of hydrocarbons and hydrocarbon radicals at 298 K and 1 bar is derived from an extensive and accurate database of 233 ab initio standard enthalpies of formation, calculated at the CBS-QB3 level of theory. The accuracy of the database was further improved by adding newly determined bond additive corrections (BAC) to the CBS-QB3 enthalpies. The mean absolute deviation (MAD) for a training set of 51 hydrocarbons is better than 2 kJ mol-1. GAVs for 16 hydrocarbon groups, i.e., C(Cd)3(C), C−(Cd)4, C−(Ct)(Cd)(C)2, C−(Ct)(Cd)2(C), C−(Ct)(Cd)3, C−(Ct)2(C)2, C−(Ct)2(Cd)(C), C−(Ct)2(Cd)2, C−(Ct)3(C), C−(Ct)3(Cd), C−(Ct)4, C−(Cb)(Cd)(C)(H), C−(Cb)(Ct)(H)2, C−(Cb)(Ct)(C)(H), C−(Cb)(Ct)(C)2, Cd−(Cb)(Ct), for 25 hydrocarbon radical groups, and several ring strain corrections (RSC) are determined for the first time. The new parameters significantly extend the applicability of Benson's group additivity method. The extensive database allowed an evaluation of previously proposed methods to account for non-next-nearest neighbor interactions (NNI). Here, a novel consistent scheme is proposed to account for NNIs in radicals. In addition, hydrogen bond increments (HBI) are determined for the calculation of radical standard enthalpies of formation. In particular for resonance stabilized radicals, the HBI method provides an improvement over Benson's group additivity method.

Ab initio group contribution method for activation energies for radical additions

M. Saeys, M-F. Reyniers, G.B. Marin, V. Van Speybroeck, M. Waroquier
AIChE Journal
50 (2), 426-444
2004
A1

Abstract 

Accurate activation energies for 67 hydrocarbon radical addition and beta-scission reactions are calculated with the CBS-QB3 ab initio method. An extension of Benson's group additivity method to activation energies is introduced. The underlying hypotheses, that is, the group concept and the additivity approximation, are validated with ab initio data. Standard activation group additivity values are obtained from the ab initio calculations for reactions involving primary, secondary, tertiary alkylic, allylic, benzylic, and vinylic radicals. The proposed group contribution method yields accurate activation energies for radical addition and for beta-scission reactions. The effect of substituents on the carbon atoms of the reactive center on the activation energy can be as large as 95 kJ/mol for the adding radical, and 187 kJ/mol for the product radical of the P-scission. Non-nearest-neighbor effects such as gauche and cis interactions have an influence of less than 3 kJ/mol per interaction on the activation energies. However, for hydrocarbons that are heavily branched near the reactive center, these interactions can become important. (C) 2004 American Institute of Chemical Engineers.

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

M. Saeys, M-F. Reyniers, G.B. Marin, V. Van Speybroeck, M. Waroquier
Journal of Physical Chemistry A
107 (43), 9147-9159
2003
A1

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 C1 to C10, 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, high-level 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.

The Kinetics of Cyclization Reactions on Polyaromatics from First Principles

V. Van Speybroeck, M-F. Reyniers, G.B. Marin, M. Waroquier
ChemPhysChem
3 (10), 863–870
2002
A1

Abstract 

Ab initio density functional theory calculations are presented on cyclization reactions of polyaromatics involved in coke formation during the thermal cracking of hydrocarbons. During coke formation, cyclization can take place at various sites, differing from each other by the local polyaromatic structure. This local structure also determines the minimum number of carbon atoms that must be added to allow the formation of a new ring. Kinetic parameters are calculated for the various ring-closure reactions by means of transition state theory. The activation energy is largely affected by the local structure of the polycyclic aromatic hydrocarbon, whereas the frequency factor varies significantly in terms of the length of the attached alkyl chain. The calculations, as presented, give a microscopic insight into the mechanisms that contribute to barrier formation and to the value of the frequency factor. The relative importance of cyclization at different sites, under conditions typical for an industrial cracking unit, is studied on the basis of the calculated rate constants at various temperatures. The results suggest that the nature of coke formation is autocatalytic: the larger the macroradicals, the faster the subsequent reactions that lead to further growth of the polyaromatic surface. This type of calculation is the first step towards the development of structural relations for the kinetic parameters in terms of the local structure of the coke matrix.

DOI 

http://dx.doi.org/10.1002/1439-7641(20021018)3:103.0.CO;2-P

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