ABSTRACT
I. THE CONCEPT OF A CHEMICAL GRAPH In chemistry graphs can represent different chemical objects: molecules,
reactions, crystals, polymers, clusters, etc. The common feature of chemical systems is the presence of sites and connections between them. Sites may be atoms, electrons, molecules, molecular fragments, groups of atoms, inter mediates, orbitals, etc. The connections between sites may represent bonds of any kind, bonded and nonbonded interactions, elementary reaction steps, rearrangements, van der Waals forces, etc. Chemical systems may be depicted by chemical graphs using a simple conversion rule:
site vertex connection ^ edge
In order to simplify the handling of molecular graphs, hydrogen-suppressed graphs,^ i.e., graphs depicting only molecular skeletons without hy drogen atoms and their bonds, are often used. They are also called skeleton graphs.'^ The hydrogen-suppressed graphs are almost universally used in chemical graph theory, because the neglect of the hydrogen atoms and their bonds in most cases cannot be the cause of any ambiguity. The hydrogensuppressed graphs corresponding to butane and cyclobutane are given in Figure 2.
