Double Ionization of Polycyclic Aromatic Nitrogen Heterocycles: Simulations on Phenanthridine

A relevant class of molecules is polycyclic aromatic nitrogen heterocycles (PANHs), which are important for the astrochemistry and biochemistry fields. Recently, three new nitrogen-bearing polycyclic aromatic molecules, cyanopyrene, cyanoacenaphthylene, and cyanocoronene, were detected in Taurus Molecular Cloud 1 (TMC-1) by radio astronomy, reinforcing the current relevance of this type of system. It is well-known that different charge states affect the structure and properties of these systems. Thus, we performed an automated search for structures and relative stabilities of the phenanthridine dication, a PANH molecular prototype. Also, computations were performed for phenanthrene dication for comparison and benchmarking reasons. Infrared spectra simulations were performed for both systems, and the main changes in spectral profile were discussed. We found that a huge isomerization reaction should occur after the double ionization process. The lowest-energy isomers of phenanthridine dication are protonated nitriles, i.e., the CNH group is formed bonded to an acenaphthene-like ring. Remarkably, this structure is very different when compared to PAH dications. The main bands in the IR spectra are located between 2000 cm^-1 and 2500 cm^-1 due to C-N stretching mode and around 3500 cm^-1 due to N-H stretching mode. Finally, simulated dipole moments of low-energy isomers are very high, from 3.80 to 7.81 D, encouraging the detection of these species by radio astronomy.