Reaction rate prediction via group additivity, part 2: H-abstraction from alkenes, alkynes, alcohols, aldehydes, and acids by H atoms

The objective of this series of investigations is to develop procedures for predicting thermodynamically consistent generic rate rules for abstraction , addition, and isomerization reactions based on state-of-the-art quantum c hemical calculations. This paper presents generic rate rules for H-abstract ion from alkenes, alkynes, alcohols, aldehydes, and acids by hydrogen atoms . As described in detail in the first paper of this series {Sumathi, R.; Ca rstensen, H.-H.; Green, W. H., Jr. J. Phys. Chem., in press}, we attempt to describe reaction rates in terms of group additivity. Analysis of ab initi o computed transition structures of a series of molecules of a given reacti on class reveals the existence of a nearly constant "reactive moiety". We e xpress thermodynamic contributions of these reactive moieties, which we ref er to as "supergroups" since they contain several polyvalent atoms, to the entire transition state species in terms of group additivity values. The gr oup additivity value of each "supergroup" is found to be transferable from one molecule to another within a given reaction family and is therefore ide ntified as the characteristic of a given reaction class. The present study in combination with Benson's group additivity tables allows prediction of r eaction rates for 15 sets of reactions, which can be used as reasonable est imates in constructing large kinetic models. When available, we compare our estimates with literature data and find good or reasonable agreement. We a lso analyze the predicted thermodynamic properties for reactants and radica ls to provide additional evidence for the reliability of the calculations. Some very small non-nearest-neighbor substituent effects are seen in the ca lculations, but these are generally too small to be easily discernible from experimental data.