<p>Naphthalene, (Aromatic stabilization Energy;
ASE, 50-60 kcal/mol) polyacenes and graphene are considered aromatic. Existing models for polyacenes predict a
linearly increasing ASE and give little insights into their high reactivity and
decreasing stability. Graphene’s
aromaticity has been studied earlier qualitatively suggesting alternate Clar’s
sextet and two-electrons per ring, but ASE estimates have not been reported yet. In this paper, various Heat of Hydrogenation
(HoH) and isodesmic schemes have been proposed and compared for the estimation
of naphthalene ASE. Results show that HoH
schemes are simple to design, are equivalent to isodesmic schemes, and unconjugated
unsaturated reference systems predict ASE values in agreement with literature
reports. Partially aromatic reference systems
underestimate ASE. HoH schemes require
calculations for a smaller number of structures, and offer scope for
experimental validation, and involve enthalpy differences. Polyacene (X-axis extensions of benzene) ASE estimates
(using HoH scheme) correlate well with experimental instability data and offer new
physical insights explaining the absence of arbitrarily larger polyacenes. ASEs extrapolated from quadratic and
logarithmic regression models have been used to estimate the largest polyacene
with limiting ASE values. ASE values for
Pyrene (Y-axis extension of benzene) and higher analogues (here called
pyrene-vertacenes) are estimated using HoH schemes. Further truncated graphene models and
graphene are approximated as combinations of polyacene and pyrene-vertacene units. First ever ASE and molecular sizes (22-255
nM) estimates predict nanometer size ranges for flat graphene in agreement with
recent experiments and offer new physical insights. These ASE and size estimates for graphene may
prove useful in the design of novel energy (hydrogen) storage, electronic and
material science applications.</p>
Relativity or aromaticity? A first-principles perspective of chemical shifts in osmabenzene and osmapentalene derivatives
