HCNCC, the exotic linear rearrangement of cyanoacetylene, is a probable interstellar molecule. The Letter compares CCSD(T)/aug-cc-pVTZ energies of five cyanoacetylene isomers, and predicts properties of the HCNCC ground singlet electronic state: geometry, equilibrium rotational constant, electric dipole moment and vibrational frequencies - based on the coupled-clusters and density functional theory (B3LyP/aug-cc-pVTZ) studies. (C) 2003 Elsevier Science B.V. All rights reserved.