Understanding the Nature of the CH···HC Interactions in Alkanes.

To understand the dispersion stabilization of hydrocarbons in solids and of encumbered molecules, wherein CH···HC interactions act as sticky fingers, we developed here a valence bond (VB) model and applied it to analyze the H···H interactions in dimers of H2 and alkanes. The VB analysis revealed two distinct mechanisms of "dispersion." In the dimers of small molecules like H-H···H-H and H3CH···HCH3, the stabilization arises primarily due to the increased importance of the VB structures which possess charge alternation, e.g., C(+)H(-)···H(+)C(-) and C(-)H(+)···H(-)C(+), and hence bring about electrostatic stabilization that holds the dimer. This is consistent with the classical mechanism of oscillating dipoles as the source of dispersion interactions. However, in larger alkanes, this mechanism is insufficient to glue the two molecules together. Here, the "dispersion" interaction comes about through perturbational mixing of VB structures, which reorganize the bonding electrons of the two interacting CH bonds via recoupling of these electrons to H···H, C···C, and C···H "bonds." Finally, an attempt is made to create a bridge from VB to molecular orbital (MO) and local pair natural-orbital coupled electron pair approximation (LPNO-CEPA/1) analyses of the interactions, which bring about CH···HC binding.