An Electrotopological-state Index for Atoms in Molecules

A new method for molecular structure description is presented in which both electronic and topological characteristics are combined. The method makes use of the hydrogen-suppressed graph to represent the structure. The focus of the method is on the individual atoms and hydride groups of the molecular skeleton. An intrinsic atom value is assigned to each atom as I = (delta v + 1)/delta, in which delta v and delta are the counts of valence and sigma electrons of atoms associated with the molecular skeleton. The electrotopological-state value, Si, for skeletal atom i is defined as Si = Ii + delta Ii, for second row atoms, where the influence of atom j on atom i, delta Ii, is given as sigma(Ii-Ij)/rij2; rij is the graph separation between atom i and atom j, counted as the number of atoms. The characteristics of the electrotopological state values are indicated by examples of various types of organic structures, including chain lengthening, branching, heteroatoms, and unsaturation. The relation of the E-state value to NMR chemical shift is investigated for a series of alkyl ethers. The E-state oxygen value gives an excellent correlation with the 17O NMR: r = 0.993 for 10 ethers. A biological application of the E-state values in QSAR analysis is given for the binding of barbiturates to beta-cyclodextrin.