  Naphthalene (C10H8) is the simplest polycyclic aromatic hydrocarbon (PAH) and
an important component in a series of astrochemical reactions involving
hydrocarbons. Its molecular charge state affects the stability of its isomeric
structures, which is specially relevant in ionised astrophysical environments.
We thus perform an extensive computational search for low-energy molecular
structures of neutral, singly, and multiply charged naphthalene and its isomers
with charge states +q = 0-4 and investigate their geometric properties and
bonding situations. We find that isomerisation reactions should be frequent for
higher charged states and that open chains dominate their low-energy
structures. We compute both the scaled-harmonic and anharmonic infrared spectra
of selected low-energy species and provide the calculated scaling factors for
the naphthalene neutral, cation, and dication global minima. All simulated
spectra reproduce satisfactorily the experimental data and, thus, are adequate
for aiding observations. Moreover, the potential presence of these species in
the emission spectra of the circumnuclear regions of active galactic nuclei
(AGNs), with high energetic X-ray photon fluxes, is explored using the
experimental value of the naphthalene photodissociation cross-section,
\sigma_{ph-d}, to determine its half-life, t_{1/2}, at a photon energy of 2.5
keV in a set of relevant sources. Finally, we show that the computed IR bands
of the triply and quadruply charged species are able to reproduce some features
of the selected AGN sources.
