We investigate theoretically the geometric structure and vibrational properties of complexes of polyenes with aluminum atoms that constitute model systems for the species formed at the interface between aluminum and polyacetylene. The calculations are performed with two quantum-mechanical techniques : ab initio Hartree-Fock and density functional theory in the local spin density approximation. These methods are first applied to a polyene molecule, all-trans octatetraene, and the calculated vibrational spectra are compared to existing experimental and theoretical data. The molecule is then made to interact with two aluminum atoms in various configurations. Since the metal atoms form covalent bonds with carbon atoms in the central part of octatetraene, strong geometric modifications occur along the conjugated system, which in turn deeply affect the vibrational spectra. These results allow us to derive the expected infrared signature of the chemical species present at the interface.