Computational analyses of the vibrational spectra of fentanyl, carfentanil and remifentanil

•The infrared (IR) spectra of fentanyl, carfentanil and remifentanil, and protonated salts, are computed using quantum chemistry methods.•New experimental FTIR spectra of these molecules are reported and compared to the calculations.•Gas phase IR spectra are calculated for both the neutral and protonated molecules.•Key vibrational modes are selected using a vibrational mode locality method of calculation.•The main contributing atomic movements in the vibrational modes are identified.