An alternation of the equilibrium structure symmetry of Li[C n]1 (n = 7-12) for different n and the correlation with boundary MO features of isolated hydrocarbons [Cn]1

The equilibrium geometries of the cylinder‐like lithium hydrocarbon complexes Li[C_n]₁ (n = 7–12) are calculated within density functional theory with PBE0 exchange‐correlation potential. The 6‐31 G* basis was set on carbon and lithium atoms while STO‐3G one was chosen for hydrogens. The complexes with odd n possess C_s symmetry for equilibrium structures, and lithium is located near the border of π‐electron system of the hydrocarbons. For complexes Li[C_n]₁ with even n (n = 8, 10, 12), the lithium position at the center of benzene ring is stable, and the equilibrium geometry has C_2v point symmetry. We have observed correlation between the alternation of the equilibrium structure of the complex and degeneracy of boundary MO of isolated hydrocarbons. By using C_nh symmetry, the π‐MO of the [C_n]₁ are found analytically within Huckel model. The nonbonding orbitals with energy α take place only for hydrocarbons with even n. These MO's are characterized by the density distributions among the terminal carbon atoms with sign alternation of AO's contributions. The solution with energy α with analogous distributions is present for larger hydrocarbons [C_n]_q with any finite q and even n. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007