TOPOLOGICAL INDICES – WHY AND HOW

By means of presently available high-level computational methods, based on quantum theory, it is possible to determine (predict) the main structural, electronic, energetic, geometric, and thermodynamic properties of a particular chemical species (usually a molecule), as well as the ways in which it changes in chemical reactions. When one needs to estimate such properties of thousands or millions of chemical species, such high-level calculations are no more feasible. Then simpler, but less accurate, approaches are necessary. One such approach utilized so-called “topological indices”. According to IUPAC ‘s definition [Pure Appl. Chem. 69 (1997) 1137]: A topological index is a numerical value associated with chemical constitution for correlation of chemical structure with various physical properties, chemical reactivity or biological activity. In the first part of the lecture, we show that „numerical values“are associated with many other complex phenomena, encountered in various areas of human activity, implying that „topological indices“ are used far beyond chemistry. Next, we discuss the number of possible chemical compounds. Simple calculation shows that the number of possible compounds zillion times exceeds the number of those that have been experimentally characterized. Even worse, in the entire Universe, there is not enough matter to make at least a single molecule of each possible compound. In the second part of the lecture, a few most popular topological indices will be presented, as well as the way in which these can be (and are being) applied in treating real-world problems.