Polyaromatic hydrocarbons (PAHs) are ubiquitous in astrochemistry and are significant
environmental pollutants with carcinogenic properties. Their intermolecular interactions
with water are relevant in the context of astrochemistry and environmental impacts of PAHs.
Clusters of PAHs and water can serve as microscopic model systems for the study of for water
binding to graphenic materials. This work employs infrared spectroscopy to investigate small
gaseous clusters containing anionic PAHs with varying shapes and sizes, providing insight
into binding motifs governing the hydration of charged graphenic surfaces. The study of
size-selected cluster anions in vacuo allows precise control over the cluster constituents,
enabling a stepwise examination of the hydration process. Conducting experiments in vacuo
eliminates bulk-phase effects and the associated spectroscopic complications, allowing for a
more detailed analysis of specific intermolecular interactions.
Infrared photodissociation spectra were obtained for anionic hydrated pyrene, tetracene,
and perylene and were interpreted using quantum chemical calculations. The intermolecular
interactions between water molecules and PAHs are encoded in the OH stretching vibrations.
Water molecules form hydrogen bonds to the excess charge, which is delocalized within the
PAH’s π-system. When multiple water molecules hydrate the PAH, additional hydrogen
bonds form between the water molecules themselves. These water subclusters can adopt
various configurations on the PAH’s surface, which can be populated either through kinetic
trapping during the formation of the clusters or by thermal population, depending on the
internal energy of the cluster anion. The OH stretching spectra provide evidence of dynamic
effects, where water molecules explore the configurational landscape of the complex. When
three or four water molecules hydrate the PAH anion, they predominantly arrange into a
ring-like structure. In this configuration, each water molecule establishes one π-hydrogen
bond with the PAH’s π-system and one hydrogen bond within the water ring. The strength
of these hydrogen bonds is reflected in the OH stretching vibrations, and their partial localization
in certain OH groups reveals that intermolecular interactions between water molecules
are stronger than those with the excess charge in the PAH. As the PAH size increases, charge
delocalization occurs over a larger area, weakening π-ionic hydrogen bonding.
Beyond the OH stretching modes of hydrated PAHs, the congested CH stretching region
of anthracene, pyrene, and perylene were studied. The complex spectroscopic patterns in this
region originate from excited states with a single quantum in CH stretching modes and two
quanta in Fermi resonances with states having two quanta in in-plane CH bending modes.
The spectra were analyzed using second-order vibrational perturbation theory calculations.