Low temperature synthesis of three cyanocyclopentadiene isomers and their role in interstellar cyano PAH formation

The recent detection of polycyclic aromatic hydrocarbons and their cyano derivatives in the Taurus Molecular Cloud challenges existing models of their formation and survival under such harsh conditions. Cyanocyclopentadienes have emerged as fundamental molecular building blocks for cyano-substituted aromatics, yet their formation in the interstellar medium remains unknown. Here, we report the gas-phase formation of distinct cyanocyclopentadiene isomers through the bimolecular reaction of 1,3-cyclopentadiene with the cyano radical explored through molecular beam studies and electronic structure calculations. Three cyanocyclopentadiene isomers are identified: the astronomically observed 1-cyanocyclopentadiene and 2-cyanocyclopentadiene species and the elusive 5-cyanocyclopentadiene. Remarkably, the reaction predominantly forms the unobserved 5-cyanocyclopentadiene isomer accounting for two-thirds of the products, according to dynamics-guided statistical calculations. These results identify 5-cyanocyclopentadiene as a key precursor to cyano-substituted aromatics, reveal a gap in current astronomical observations, and motivate targeted searches for this elusive molecule in cold molecular clouds. The recent detection of polycyclic aromatic hydrocarbons and their cyano derivatives in the cold Taurus Molecular Cloud challenges existing models of formation and survival under such harsh conditions. Here, the authors report the gas-phase formation of cyanocyclopentadiene isomers through molecular beam studies and electronic structure calculations.