The Correct Physical Basis of Protobranching Stabilization

The source of the extra stability of branched hydrocarbons over unbranched (hereafter referred to as protobraching stabilization) has been explained in several different self-consistent ways. Gronert, basing his arguments on well-established properties of organic molecules, namely that geminal atoms strongly repel each other, formulated a model which accounted for this "protobranching" stability simply and very well. However, careful quantum computations were found to yield correct protobranching energies only if they properly took electron correlation into account. Such a source of stability would not have been needed in Gronert's model. The present article analyzes what is correct and what is uncertain about Gronert's atomic repulsion model, and concludes that the computations of Wiberg, Bader, Grimme, Schleyer, and their colleagues overturn Gronert's model and that bond-bond electron correlation energies provide the correct explanation of protobranching.