Alkanes are molecules that contain only carbon and hydrogen atoms, connected by single bonds. Short-chain alkanes such as butane and octane — which contain linear chains of four and eight carbon atoms, respectively — stretch out in extended zig-zags. However, longer hydrocarbon chains tend to fold themselves into hairpin structures.

Ricardo Mata, Martin Suhm and their colleagues at the University of Göttingen, Germany, determined the point at which this transition becomes energetically favourable. The researchers performed spectroscopy on supersonic jets of alkane molecules at temperatures of 100–150 kelvin — and found that the folded structure becomes more stable than the extended conformation when an alkane chain is around 18–19 carbon atoms long.

The result broadly agrees with the authors' quantum calculations, and can be used to train computer models of molecular mechanics.

Angew. Chem. Int. Edn http://dx.doi.org/10.1002/anie.201202894 (2012)