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 center dot center dot center dot HC interactions act as sticky fingers, we developed here a valence bond (VB) model and applied it to analyze the H center dot center dot center dot H interactions in dimers of H-2 and alkanes. The VB analysis revealed two distinct mechanisms of dispersion. In the dimers of small molecules like H-H center dot center dot center dot H-H and H3CH center dot center dot center dot HCH3, the stabilization arises primarily due to the increased importance of the VB structures which possess charge alternation, e.g., C+H-center dot center dot center dot H+C- and C-H+center dot center dot center dot 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 center dot center dot center dot H, C center dot center dot center dot C, and C center dot center dot center dot 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 center dot center dot center dot HC binding