In the first two papers of this series (Sumathi, R.; Carstensen, H.-H.; Green, W. H., Jr. J. Phys. Chem. 2001, 105, 6910. Sumathi, R.; Carstensen, H.-H.; Green, W. H., Jr. J. Phys. Chem. 2001, 105, 8969), a procedure has been developed on the basis of ab initio quantum chemical calculations to express the generic reaction rate in terms of the thermochemical contributions of the reactive moiety ("supergroup") in the transition structure. The supergroups are derived with the assumption that the contribution from the unreactive moiety to the thermochemistry is given by its group additivity (GA) values. This paper presents the qualitative justification for partitioning the energy of the transition structure into contributions from unreactive and reactive moieties using atoms in molecule (AIM) analysis. The couplings between these moieties, if any, are studied quantitatively using quantum chemical calculations at the CBS-Q level on reactions of the type XCH2CH3 + Y --> XCH2CH2. + HY (X = H, CH3, (CH3)(2)CH, (CH3)(3)C, F, Cl, NH2, SH, OH, OCH3, OC(O)H, OC(O)-CH3, CHO, COCH3, COOH, COOCH3, CH=CH2, CCH; Y = H, CH3). The present work thus focuses on the strength and limitations of the GA procedure and explores the effects of varying electronegative and bulky non-next-neighbor substituents, which are separated from the reactive center by a CH2 group, on supergroup values. Both the C-H bond dissociation energies (BDE) and barrier heights to these reactions vary appreciably depending on the non-next-neighbor substituent, X. The preferred conformation around the XCH2- - -CH2(HY) bond in transition structures is largely determined by the effective hyperconjugative interaction between the bonds of the CH2X group and the forming radical center. The effect of X on reaction barriers is subsequently modeled through a multilinear expression that is based on its inductive, steric, and hyperconjugative parameters, suggesting a practical way to accommodate non-next-neighbor effects on generic rate rules predicted using group additivity.