Carbon-centered radical addition and β-scission reactions:: Modeling of activation energies and pre-exponential factors

A consistent set of group additive values Delta GAV 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 Delta GAV degrees s in the broad range of 300-1300K is limited to +/- 4.5 kJmol(-1) on activation energies and to +/- 0.4 on logA (A: pre-exponential factor) for 90% of the groups. Validation of the Delta GAV 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.