Intramolecular Nonbonded Attractive Interactions: 1-Substituted Propenes

Whereas cis-substituted alkenes are normally significantly less stable than the trans-isomers, there is a group of 1-substituted propenes (X = F, OMe, Cl, Br, SMe) where the cis-isomers are the more stable. The calculated structures show that there is steric repulsion with the cis-isomers. However, this is overcome by attractive Coulombic interactions when X = F or OMe and by attractive dispersive interactions when X = Cl or Br. It was possible to calculate the magnitude of the latter term via the summation of the appropriate MP2 pair energies. The calculated and observed energy differences could be reproduced by a summation of steric, electrostatic, and dispersive interactions.