Sj. Cyvin et al., CONTRIBUTIONS TO THE THEORY OF BENZENOID ISOMERS .2. BUILDING-UP PRINCIPLES, Journal of molecular structure. Theochem, 334(2-3), 1995, pp. 243-247
The fundamental building-up principle states that three types of addit
ion of hexagons are sufficient for generating all benzenoids. The prin
ciple is applied to benzenoid CnHs isomers with the result of another
principle, which implies the three addition units C4H2, C3H and C-2.