# Double Ionization of Polycyclic Aromatic Nitrogen Heterocycles: Simulations on Phenanthridine

## TL;DR

This paper studies how double ionization changes the structure and infrared spectra of a nitrogen-containing aromatic molecule called phenanthridine, which is important in astrochemistry and biochemistry.

## Contribution

The study reveals new structural isomers and IR spectral features of phenanthridine dication, distinct from typical polycyclic aromatic hydrocarbons.

## Key findings

- Phenanthridine dication forms protonated nitrile isomers after double ionization.
- Infrared spectra show strong C–N and N–H stretching modes at 2000–2500 cm–1 and 3500 cm–1.
- Low-energy isomers have high dipole moments (3.80–7.81 D), aiding radio astronomy detection.

## Abstract

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.