The IR spectrum of propyne adsorbed on Cu(111) has been analyzed using first-principles density functional periodic slab calculations. The agreement between experimental reflection absorption infrared spectra data and the present results is almost quantitative. This permits an unambiguous assignment for the band appearing at 1361 cm(-1), thus solving some controversy in the existing literature. This band was previously assigned either to the C1C2 stretching or to CH3 symmetric deformation modes whereas the present work shows that, as a matter of fact, it involves a strong coupling between the two corresponding group frequencies. This is ail important result because it shows that, to properly assign IR bands in polyatomic adsorbed molecules, the coupling between different group frequencies has to be taken into account.