RESONANCE INTERACTIONS IN ACYCLIC SYSTEMS .1. ENERGIES AND CHARGE-DISTRIBUTIONS IN ALLYL ANIONS AND RELATED-COMPOUNDS

The energies of dissociation of propane to 1-propyl cation and anion and of propene to allyl cation and anion may be satisfactorily reproduced via ab initio calculations at the MP4/6-311++G**//6-31G* level. The reaction of 1-propyl cation with propene to give the unconjugated allyl cation was found to be endothermic, whereas the corresponding reaction of the anion was exothermic. The rotational barrier for allyl cation was 36 kcal/mol, whereas that for the anion was 19 kcal/mol. These data were analyzed in terms of electron delocalization and the electrostatic energies of the ions, and it was concluded that whereas the cation had significant resonance stabilization, the anion had little stabilization.A series of allyl type anions were examined making use of 6-311++G wave functions calculated at the 6-31G geometries. Correction for electron correlation at the MP3 level led to calculated proton affinities which agreed well with the experimental values. Electronegative atoms at the central position had little affect on the proton affinities, but when they were at the terminal positions, there was a large change.The changes in electron population among the anions were studied via numerical integration of the charge densities within boundaries which may be assigned to the atoms in the ions. The more stable anions are characterized by a-+-charge distribution for the three atoms in the allylic system, leading to internal coulombic stabilization. © 1990, American Chemical Society. All rights reserved.