Influence of pi bonds on the structure of organic molecules

Date of Completion

January 2005


Chemistry, Physical




Microwave spectroscopy and ab initio calculations on the conformational isomers of 1,6-heptadiyne and allylbenzene indicate that the presence of π bonds help the molecules acquire gauche and eclipsed conformations that are disfavored by the steric repulsions in alkanes. In 1,6-heptadiyne the isomers arrange in increasing energy as GG trans < AG < AA < GG cis. We have experimentally identified the GG trans and AG conformers. The other two isomers are too high in energy or not stable enough to allow experimental identification. For allylbenzene no isomers have been experimentally identified. MP2 calculations predict the Cs structure to be more stable than the C1 isomer by less than 1 kJ/mol. The rotational barrier for the isomer interconversion is estimated to be ∼ 12 kJ/mol. We discuss experimental and theoretical evidence that suggests that the CH/π and CC/π interactions responsible for the structural preferences is mostly dispersive in nature with very little contribution from electrostatics. ^