Modeling Soot and Its Functionalization under Atmospheric or Combustion Conditions by Density Functional Theory within Molecular (Polycyclic-Aromatic-Hydrocarbon-like) and Periodic Methodologies

Graphite, and particularly defective graphite, is chosen to model soot particles. Quantum mechanical calculations
are first carried out on molecular polycyclic-aromatic-hydrocarbon-type systems and then extended to a periodic
representation of one graphite layer. The features of the interaction of H, HO, NO, NO2, and NO3 with these
model systems are examined, with the aim of defining a suitable representation of the atmospheric or combustion
gas−solid interactions by which functionalization reactions can take place. The more interesting interactions
with small reactive molecules regard the edge of the graphene sheet and the in-plane carbon vacancies. While
these interactions can be well described by sufficiently extended molecular models, periodic models are
necessary to describe accurately the equilibrium geometries because they introduce the necessary geometric
constraints. The ability of a graphene sheet to easily accommodate unpaired electrons in σ or π orbitals is the
basis for its interesting interactions with the small molecules present in tropospheric chemistry or combustion
processes.