Do defects in PAHs promote catalytic activity in space? Stone-Wales pyrene as a test case

Using density functional theory (DFT), we studied the formation of the Stone Wales defect in pyrene, as a prototype PAH molecule. In addition, we studied the reactivity of the defective and pristine pyrene toward hydrogenation, a processes that can occur in some regions of the interstellar medium. We found that the formation of the defect requires overcoming energies of the order of 8.4 eV, but the defected structure is stable due to the high reverse reaction barrier (approx 6 eV). We also found that the presence of the defect decreases the sticking barrier for the first hydrogenation and promote more stable singly and doubly hydrogenated intermediates respect to the pristine pyrene. Finally, our results show that both Stone-Wales and pristine pyrene can lead to the formation of H2 through an extraction mechanism involving H atoms attached on distal carbon atoms with energy barriers below 2 eV.