Formation of polyynes and ring-polyyne molecules following fragmentation of polycyclic aromatic hydrocarbons.

In this work, we perform molecular dynamic (MD) simulations to investigate the stability and fragmentation processes of vibrationally excited linear polycyclic aromatic hydrocarbons (PAHs). The program of CP2K in combination with the semi-empirical method PM3 is utilized for the MD simulations. The simulations show that the formation of molecular hydrogens (H2) is different than previous studies, in particular, different than compact PAHs. At high temperatures, linear PAHs tend to open aromatic rings and convert the sp3 C–C or sp2 C=C bonds to sp C≡C bonds by removing H2; i.e. polyynes are formed in such process. Besides polyynes, PAHs attached with sp-bonded polyyne chains are commonly observed at high temperatures. We notice that due to the addition of flexible tails (polyynes), the ring-polyyne molecules do not dissociate for a long period of time at high temperatures. Such structures facilitate the molecules to survive in the harsh environment of the interstellar medium. In addition, the ring-polyyne structures induce dipole moments that could, in principle, be detected by radio astronomy. [ABSTRACT FROM AUTHOR]