Alternating planarity/nonplanarity in n-doped odd-membered, all-trans polyenes: Molecular structures of NaCnHn+2 (n=3, 5, 7, and 9)

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 d istinctly different structural motifs have been identified. In one motif, t he dopant atom interacts with an allylic polyene unit in a 4 pi-electron in teraction that results in a planar polyene backbone. The other motif has th e dopant atom interacting with a pentadienyl polyene unit in a 6 pi-electro n interaction, which produces a significantly warped polyene backbone. Inde pendent of structural motif at the doping site, the portion of the polyene structure outside the interaction region remains largely undisturbed in ter ms 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 pronounce d 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 restricte d B3LYP wavefunctions are stable for all structures investigated.