We present a theoretical study of the structure and vibrations of acrylonitrile adsorbed on a Cu(100) metal surface. Simulations have been carried out by means of the density functional theory adopting periodic boundary conditions and including van der Waals dispersion forces. The two most stable structures (which are almost degenerate in energy) correspond to the molecule adsorbed parallel to the metal surface. In both geometries, the vinyl (C=C) and the cyano (C≡N) groups are the anchorage sites to the surface atoms. Low energy transition barriers allow fast isomerization between both structures. Molecular adsorption shifts some of their vibrational frequencies with respect to the gas phase, mainly the C=C and the C≡N stretching modes, in agreement with recent experimental measurements [Tornero et al., Phys. Chem. Chem. Phys., 2011, 13, 8475].