Alternating Planarity/Nonplanarity in n-Doped Odd-Membered, All-Trans Polyenes: Molecular Structures of NaC_nH_n+2 (n = 3, 5, 7, and 9)

Abstract

The electronic structures of small, odd-membered, all-trans polyenes doped with one Na atom at various positions have been investigated using Hartree-Fock and density functional (B3LYP) theory with a 6-31G(d) basis set. Two distinctly different structural motifs have been identified. In one motif, the dopant atom interacts with an allylic polyene unit in a 4π-electron interaction that results in a planar polyene backbone. The other motif has the dopant atom interacting with a pentadienyl polyene unit in a 6π-electron interaction, which produces a significantly warped polyene backbone. Independent of structural motif at the doping site, the portion of the polyene structure outside the interaction region remains largely undisturbed in terms of planarity and bond length alternation. For a particular formula unit and potential energy surface, the stationary points corresponding to minima and transition states are remarkably close in energy despite the pronounced changes that occur in the dihedral angles of the polyene backbone at the dopant sites. Whereas internal and external instabilities are found in the Hartree-Fock wavefunctions for NaC₇H₉ and NaC₉H₁₁ structures, the restricted B3LYP wavefunctions are stable for all structures investigated.