The Physical Meaning of Topological Indices

Abstract A handicap in using topological indices for QSAR is the lack of adequate provisions for specifying the nature of graph vertices (i.e. accounting for heteroatoms) or of graph edges (i.e. accounting for multiple bonds). Analysis of seven topological indices (i.e. χ, J, w, I DW, IDW, I DW) indicates that χ-type indices mimic the molecular van der Waals volume, while (within series of isochoric molecules) the other indices mimic the molecular van der Waals area. These results indicate a physically relevant approach to account for heteroatoms and multiple bonds within the topological index framework, namely, fitting the equation defining the index at hand to calculated molecular van der Waals volumes or areas. A computer program (in Pascal) for calculating these indices is also provided.