The IR spectra of 5-bromo-2,4-pentadiynenitrile (Br-C≡C-C≡C-CN) and 2,4-hexadiynenitrile (CH3-C≡C-C≡C-CN), a compound of interstellar interest, have been recorded within the 4000-500 cm-1 spectral region and calculated by means of high-level ab initio and density functional theory (DFT) calculations. Although the calculated structures of both compounds are rather similar, there are very subtle differences, mainly in the strength of the C≡C bond not directly bound to the substituent. These subtle bonding differences are reflected in small, but not negligible differences in the electron density at the corresponding bond critical points, and, more important, reflected in the IR spectra. Indeed the IR spectrum for the bromine derivative presents two well differentiated strong bands around 2500 cm-1, whereas for the methyl derivative both absorptions coalesce in a single band. These bands correspond in both cases to the coupling between C≡C and C≡N stretching displacements. A third, very weak band also associated with C≡C and C≡N coupled stretches, is observed for the bromine derivative but not for the methyl one, due to its extremely low intensity.