On a topological interpretation of electronic and vibrational molecular energies

A relationship between Randic's connectivity index and various quantum mechanical parameters derived from the Huckel Molecular Orbital (HMO) approach is demonstrated. When applied to conjugated hydrocarbons, this index represents the measure of the global pi electron molecular energy and, therefore, of the resonance energy. Moreover, the development of the procedure, allows the introduction of a new definition of the bond order which, in turn, makes possible a better prediction not only for bond lengths of naphtalene but also for the resonance integral and conjugation energy for butadiene. Also, a corrected value for the Randic index is deduced, which allows for the reduction of the discrepancies between the spectroscopic and thermodynamic values for the resonance integral, the significant improvement in the prediction of the resonance energies as well as the accurate prediction of the vibrational C=C energies for different substituents of the carbon atoms. Moreover, the use of the topological charge indices [Galvez, J., Garcia, R., Salabert, M.T., Soler, R., J. Chem. Inf. Comput. Sci. 34 (1993) 520-525] [1] (TCl), leads to a remarkable improvement in the prediction of the resonance energies of a set of non-alternant hydrocarbons in a cross-validation test. All these results reinforces the hypothesis, previously mentioned by us, that molecular topology may be considered as an alternative and independent way to describe chemical structure. (C) 1998 Elsevier Science B.V.