A chemical index inspired by biological plastic evolution: valence-isoelectronic series of aromatics.

Plastic evolution is a new perspective of the evolutionary process conjugating the effect, on one hand, of the acquired characters and, on the other, of the relationships that emerge among the principle of evolutionary indeterminacy, the principle of morphologic determination, and natural selection. Plastic evolution is applied to design the coordination index Ic. Ic is used to characterize the valence-isoelectronic series of cyclopentadiene, benzene, toluene, and styrene and compared to charge indices for dipole moment. The parameters needed to calculate Ic are minus the standard enthalpy of formation T, molecular surface area S, and molecular weight W. With the exception of the O heteromolecule, Ic allows differentiating quantitatively the styrene series as well as the cyclopentadiene from the toluene series. Ic improves the multivariable regression equations for dipole moment and is a predictive index when it is used together with the topological charge indices. The correction introduced in the dipole moment is produced in the correct direction. Inclusion of the heteroatom in the pi-electron system is beneficial for the description of the dipole moment, owing to either the role of additional p and/or d orbitals provided by the heteroatom or the role of steric factors in the pi-electron conjugation. Principal component analysis groups the chemical indices in three classes, which are in agreement with cluster analysis.