Chemists have known for almost 80 years that branched alkanes are more stable than their unbranched isomers. There have been many explanations for this “branching effect” that differentiates the n-alkanes from their iso and neo counterparts.
William McKee and Paul von Ragué Schleyer, University of Georgia, Athens, GA, USA, have built on their earlier protobranching model to explain why branched alkanes are tougher than their straight chain isomers. Their theory computes the electronic correlation between the interactions of 1,3-alkyls with another alkyl group in a given alkane. The branching, it seems, means that the electronic structure is simply more compact and this decreases molecular surface area per atom and so leads to a lowering of energy and a concomitant increase in stability.
The team carried out the necessary geometry optimizations and harmonic frequency computations using the Gaussian 2009 program, they calculated molecular surface area using Chimera and harmonic frequencies in GAMESS.
- Correlation Effects on the Relative Stabilities of Alkanes,
William C. McKee, Paul von Ragué Schleyer,
J. Am. Chem. Soc. 2013.