The energy spectra of a class of 2-D polybenzenoid Clar's hydrocarbons with
a large number N(N similar to 10(4)) of carbon atoms is studied theoretica
lly. It is shown that at the asymptotic case N --> infinity the energy gap
(EG) Delta E (N --> infinity) is different from zero, i.e., the pi-systems
should possess semiconductor properties. The results for the EG Delta E(N -
-> infinity) not equal 0 of the hydrocarbons are in qualitative agreement w
ith the results for the :EG calculated for a class of I-D ladder polymers h
aving the same edge structure as the hydrocarbons. With increasing the numb
er (M) of the pi-centers of the elementary unit of the polymers, the band g
ap Delta E(M --> infinity) approaches also to a value different from zero.
The quantitative results on the equlibria geometries of the hydrocarbons an
d the polymers correspond to Clar's qualitative characterization of benzeno
ids composed of disjoint "pi-sextets". The energetics of the hydrocarbons w
ith different types of defects and the corresponding Tamm and Frenkel state
s were also investigated.