On the infrared spectra of <bold>HC_{2<italic>n</italic>+1}N</bold> families and their cations

The HC2n+1N families are important components of the interstellar molecules, and are also proposed as the potential carriers of diffuse interstellar bands (DIBs). Therefore, they are of great astrophysical significance. They have been detected through radio emission. However, the weak intensity of these emissions has restricted their identification. We aim to study their infrared emission theoretically and therefore, supply new clues for their identification. We systematically calculated the infrared vibrational spectra through quantum chemical approaches for the neutral and cationic HC2n+1N (n=[0, 8]). We employed four methods, including B3LYP/6-31G*, B3PW91/aug-cc-pVTZ, B3PW91/cc-pVTZ, and CCSD/cc-pVTZ. We compared the calculated and experimental spectra of HC3N to check the accuracy of our methods. We find that the CCSD/cc-pVTZ method provides the best accuracy with respect to the experiment. For HC2n+1N molecules, short neutral chains (with n 2n+1N neutrals is located in the range of 2230.9 to 2278.8 cm−1, which differs from other CN-substituted molecules, and therefore could be the possible characteristic bands for the identification of such molecules.