1,3 Geminal Interactions as the Possible Trend Setting Factors for C−H and C−C Bond Energies in Alkanes. Support from a Density Functional Theory Based Bond Energy Decomposition Study

Abstract

A bond energy decomposition analysis has been carried out to rationalize the well-established experimental fact that C−C and C−H bond energies decrease with increasing substitution on the carbon. It is shown that this trend is set by steric 1,3 repulsive interactions (geminal repulsion) that increase in the order 1,3 hydrogen−hydrogen < 1,3 hydrogen−carbon < 1,3 carbon−carbon. On the other hand, the radical stabilization energy has little influence on the observed trend for the C−H bond energy in H−CR₁R₂R₃ or the C−C bond energy in H₃C−CR₁R₂R₃. Thus, it varies in H−CR₁R₂R₃ from −7.2 kcal/mol (H−CH₃) to −6.5 kcal/mol (H−C(CH₃)₃) and in H₃C−CR₁R₂R₃ from −19.0 kcal/mol (H₃C−CH₃) to −16.9 kcal/mol (H₃C−C(CH₃)₃). It was further found that the average intrinsic C−H bond energy in H−CR₁R₂R₃ of 129.2 kcal/mol is smaller than the average intrinsic C−C bond energy in H₃C−CR₁R₂R₃ of 143.4 kcal/mol. However, after the inclusion of steric effects, the overall C−H bond becomes stronger than the C−C bond. The role of steric 1,3 repulsive interactions as the trend setting factor has most recently been suggested by Gronert (J. Org. Chem.2006, 71, 1209) based on an empirical fit of alkane atomization energies.