Abstract
A semiempirical method, the PM3-family-correlation (PM3-FC) method, has been developed to estimate the activation energies for hydrogen abstraction reactions between hydrocarbon radicals and hydrogen donors. The method combines semiempirical PM3 calculations of transition state enthalpies [ΔH°f(TS)] with family correlations between the PM3-calculated and experimental ΔH°f(TS) values on the basis of regression analysis. For a test set of 40 hydrogen abstraction reactions, including alkyl + alkyl−H, alkyl + allylalky−Hα/arylalkyl−Hα, and benzyl + allylalkyl−Hα/arylalkyl−Hα, it is found that the deviations between the PM3-calculated and experimental ΔH°f(TS) values are systematic and dependent on the transition state structures. The structurally homologous transition states show excellent linear correlation. Using the obtained linear regression parameters to scale the PM3-calculated ΔH°f(TS) values leads to a very significant increase in estimation accuracy. The activation energies for the whole test set are...
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