Alkyl chains with CN and CCH substituents prefer gauche conformations

Microwave spectroscopy and first principles calculations of the conformational isomers of 1,6-heptadiyne indicate that the presence of π bonds help the molecules acquire gauche conformations that are disfavored by steric repulsion in alkanes. The isomers arrange in increasing energy as GGtrans < AG < AA < GGcis. We have experimentally identified the GGtrans and AG conformers. The other two isomers are too high in energy or not stable enough to allow identification under our experimental conditions. We discuss experimental and theoretical evidence that suggests that the CH/π interaction responsible for the structural preference is mostly dispersive in nature with very little contribution from electrostatics.