Resonance interactions in acyclic systems

Abstract

The role of resonance in the stabilization of allyl ions has been studied via ab initio molecular orbital calculations using the 6-311++G basis set and correction for electron correlation. The rotational barrier in allyl cation (35 kcal/mol) is composed of electron delocalization and electrostatic effects whereas the barrier in allyl anion (20 kcal/mol) appear to largely result from electrostatic terms. Allyl anions are stabilized by electronegative groups at the terminal positions, but not at the central position. The changes in atom energies on ionization were calculated and the change in energy of the hydrogen accounts for about half of the difference in gas phase acidity between alcohols and carboxylic acids. The conformations of butadiene were studied, and evidence was obtained for the second rotamer being gauche with a torsional angle on the order of 35°. The origin of the rotational barriers in butadiene and acrolein are discussed. © 1989 IUPAC