  The study continues the previous development [MATCH, 72 (2014) 39-73] of the
perturbative approach to relative stabilities of pi-electron systems of
conjugated hydrocarbons modeled as sets of weakly-interacting initially-double
(C=C) bonds. Distinct isomers of acyclic hydrocarbons (polyenes) are now under
focus. The relevant total pi-electron energies (E) are expressed in the form of
power series containing members (E_(k)) of even orders (k=0,2,4,...) with
respect to the averaged resonance parameter of initially-single (C-C) bonds.
Terms to within the sixth order (k=6) inclusive are shown to be of importance
for discrimination between similar isomers. In this connection, missing
expressions for corrections E_(6) are originally derived. Conjugated paths of
various lengths (i.e. linear chains consisting of C=C and C-C bonds
alternately) are shown to be the most important (but not the only) fragments
contributing to stabilization of any acyclic pi-electron system. Again, new
types of fragments (substructures) are revealed (viz. the so-called composite
conjugated paths) that contribute to destabilization of the system concerned.
As a result, formation of the total energy of an acyclic pi-electron system is
concluded to be governed by an interplay between stabilizing and destabilizing
factors. Accordingly, the perturbative approach applied offers us an extension
of the concept of conjugated paths. Particular isomers containing four, five
and six C=C bonds are considered in a detail as examples.
