Coupled-clusters CCSD(T)/ce-pVTZ calculations have been carried out on several lowest energy isomers of cyanodiacetylene H-CC-CC-CN, previously selected in an extensive DFT study. Apart from the izonitrile H-CC-CC-NC1 branched species CC(H)CCCN and CCCC(H)CN are found to be of particular importance, less stable than cyanodiacetylene by 48.5 and 51.0 kcal/mol, respectively. Harmonic frequencies and IR absorption intensities of vibrational fundamentals were predicted at the B3LYP/a.ug-cc-pVTZ level. (c) 2006 Elsevier B.V. All rights reserved.