Exotic isomers of dicyanoacetylene: A density functional theory and ab initio study

Prospects for the existence and detection of yet unknown dicyanoacetylene (NCCCCN) isomers are discussed, based on quantum-chemical calculations for linear, hexagonal and branched C4N2 structural variants. It is concluded that apart from dicyanoacetylene itself and its two already discovered isomers, NCCCNC and CNCCNC, at least two other species are of importance: linear CCCNCN and Y-shaped CC(CN)CN (dicyanovinylidene). Combined CCSD(T) and MP4 calculations predict CC(CN)CN and CCCNCN to be 57 kcal/mol and 66 kcal/mol less stable than dicyanoacetylene, respectively. The height of the energy barrier for dicyanoacetylene<--dicyanovinylidene isomerization is about 5 kcal/mol. Density functional theory calculations indicate that CCCNCN should give rise to prominent IR absorption bands, two orders of magnitude stronger than those of dicyanoacetylene. (C) 2002 American Institute of Physics.