The aim of this paper is to show that the ZEP topological index has better discrimination power than four well known topological indices in molecular chemistry: Balaban index, Harary index, Randic index, and Wiener index.

The molecular hydrophobicity (RMO) of several newly synthesized phenoxathiin derivatives and of phenols with congeneric structures have been recently correlated with some simple physico-chemical calculated parameters of compounds: the water solubility (log Sw); the partition coefficient (log P); the Gibbs energy of formation (∆Gf ), and the aromaticity index (HOMA) [Beteringhe, A., Radutiu, A. C., Constantinescu, T., Patron, L. and Balaban, A. T., Quantitative Structure-Property Relationship (QSPR) study of the hydrophobicity of phenols and 2-(aryloxy-α-acetyl)- phenoxathiin derivatives, Rev. Chim. (Bucures¸ti) , 59 (2008), No. 11, 1175–1179]. The best correlation was found as a biparametric regression equation in terms of log Sw and HOMA, which cannot be improved by adding one or two of the parameters aforementioned. In the present work we describee the weighted electronic distance based topological index (ZEP) and then use it for QSPR studies of RMO in combination with log Sw, log P, ∆Gf and HOMA. Most of the three parameter QSPR correlations of RMO are significantly improved by involving the theoretical parameter ZEP.

The octane number (ON, MON and PON) for the molecular structures of 18 octane isomers have been correlated using the quantitative structureproperty relationship (QSPR) method, with topological index SD. For single parameter correlation the index SD shows poor results (RON, r = 0.406; MON, r = 0.490; PON, r = 0.448), whereas for two-parameter correlation almost any combination among the above DC was found to give relatively high r value. The best correlation coefficients are as follows: for RON, r = 0.993; MON, r = 0.968; PON, r = 0.985. For RON, the best model obtained by our regression analysis is RON = −227.218 + 7.63 ∗ SD − 37.111 ∗ DC , with r = 0.993, s = 4.8, F = 534