Carbon‐Centered Radical Addition and β‐Scission Reactions: Modeling of Activation Energies and Pre‐exponential Factors

Abstract

A consistent set of group additive values DeltaGAV degrees for 46 groups is derived, allowing the calculation of rate coefficients for hydrocarbon radical additions and beta-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 DeltaGAV degrees 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 DeltaGAV degrees 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.

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

“…The behavior of the polar factor, F, versus the energy of the charge separated configurations allows evaluating the nature of the polar influence. In contrast to the variation of adding radicals for carbon-centered radical additions, [29] there is no correlation found between the expected nucleophilicity (Ei,H -Eea,alkene) and the polar factor F. As illustrated in Figures 5 and 6, the expected electrophilicity (Ei,alkene -Eea,H) correlates with the polar factor F for both sets of hydrogen additions reactions in accordance with the charge transfer to the hydrogen radical in the transition state as identified by Clarke et al [43] In general, the electrophilic influence is much smaller for the hydrogen addition to the substituted C2 atom (Table 4) than for the addition to the unsubstituted C2 atom, but a quantitative correlation of this effect that is valid for all of the reactions in Tables 3 and 4 is not straightforward. Similar observations concerning the use of a combination of an Evans-Polanyi relation and polar factors describing the reactivity trends in carbon radical centered radical additions have been made previously for carboncentered radical additions.…”

“…As a measure for the deviation the factor of deviation, ρ, as applied in previous studies, [29,61,62] is defined as follows:…”

“…[29] The computational approach involves conventional transition state theory based on high-level CBS-QB3 ab initio calculations, [52] which has already shown its accuracy for similar radical reactions in previous work. [27][28][29]53] In this paper, first the computational method is validated for hydrogen additions by comparing the CBS-QB3 rate coefficients with computational experimental data available in literature. Next, rate coefficients are calculated for 34 reactions, from which 33 group additive values are derived, and a model to correct for tunneling effects is proposed for rate predictions at temperatures below 1000K.…”

“…Quantum chemical calculations can be applied to any reaction type, and extracting quantitative values of rate coefficients does not rely on assuming a reaction scheme. Therefore, the use of quantum chemistry to calculate rate coefficients for gas phase radical reactions is particularly attractive and the more recently developed parameterization schemes for carbon centered radical additions and β scissions [29] and for hydrogen abstraction [22][23][24][25][26] are all based on first principles calculations to determine the model parameters.Although hydrogen additions are less commonly studied than hydrogen abstractions or non-hydrogen radical addition, rate coefficients for this reaction family are included in many literature reviews on radical reaction rate coefficients. In general, these reviews report not only experimental data but also predicted and extrapolated data.…”

“…Moreover, both methods are limited to predictions of the activation energy and do not allow prediction of the pre-exponential factor. To the best of our knowledge, no structure-reactivity correlation covering a wide range of hydrogen additions to hydrocarbons is available in literature.This work aims at the determination of a consistent set of group additive values (∆GAV°) for the prediction of activation energies and pre-exponential factors of hydrogen additions to a broad range of hydrocarbons and the reverse C−H β scissions, in line with the recently reported group additive model for carboncentered radical addition.[29] The computational approach involves conventional transition state theory based on high-level CBS-QB3 ab initio calculations, [52] which has already shown its accuracy for similar radical reactions in previous work.[ [27][28][29]53] …”