Donna J. Nelson

A MNDO study of the structures and stabilities of some substituted pentadienyl anions

Abstract A MNDO study of the structures and stabilities of a series of pentadienyl anions has predicted that the W geometric form is more stable than the S or U forms by about 1.0 and 3.7 kcal/mol, respectively. Substitution of methyl for hydrogen in the 1- and 3-positions of pentadienyl anions has been found to stabilize the anion but less effectively than in the cations. Stabilization of the anion by alkyl (∼1–2 kcal/mole) was less than that observed upon substitution of phenyl (∼11 kcal/mole). Polarization, rather than negative hyperconjugation or induction, is probably the most important stabilizing factor. Charge density in the pentadienyl anion and cation follows the order: 3-> 1-> 2-positions. Charge alternation increases significantly upon substitution so that the same order of charge densities is found, with significantly more negative charge borne at the 3- and 1-positions in substituted anions.