Toward a Generalized Hückel Rule: The Electronic Structure of Carbon Nanocones

In this work, we
investigate a particular class of carbon nanocones,
which we name graphannulenes, and present a generalized Hückel
rule (GHR) that predicts the character of their ground state based
on simply the three topological indices that uniquely define them.
Importantly, this rule applies to both flat and curved systems, encompassing
a wide variety of known structures that do not satisfy the “classic”
4n + 2 rule such as coronene, corannulene, and Kekulene.
We test this rule at the Hückel level of theory for a large
number of systems, including structures that are convex and flat,
with a saddle-like geometry, and at the CASSCF level of theory for
a selected representative subset. All the performed calculations support
the GHR that we propose in this work.