No abstract

A new possibility for estimating the octanol/water coefficient (log P) was investigated using only one descriptor, the semi-empirical electrotopological index (ISET). The predictability of four octanol/water partition coefficient (log P) calculation models was compared using a set of 131 aliphatic organic compounds from five different classes. Log P values were calculated employing atomic-contribution methods, as in the Ghose/Crippen approach and its later refinement, AlogP; using fragmental methods through the ClogP method; and employing an approach considering the whole molecule using topological indices with the MlogP method. The efficiency and the applicability of the ISET in terms of calculating log P were demonstrated through good statistical quality (r > 0.99; s < 0.18), high internal stability and good predictive ability for an external group of compounds in the same order as the widely used models based on the fragmental method, ClogP, and the atomic contribution method, AlogP, which are among the most used methods of predicting log P.

The semi-empirical electrotopological index, I SET , used for quantitative structure-retention relationship (QSRR) models firstly developed for alkanes and alkenes, was remodeled for organic functions such as ketones and aldehydes. The I SET values for hydrocarbons are calculated through the atomic charge values obtained from a Mulliken population analysis using the semi-empirical AM1 method and their correlation with the SET i values attributed to the different types of carbon atoms according to experimental data. For ketones and aldehydes the interactions between the molecules and the stationary phase are slowly increased relative to the hydrocarbons, due to the charge redistribution that occurs in the presence of heteroatoms. For these polar molecules the increase in the interactions was included in the calculation of the I SET values through the dipole moment of the whole molecule and also through an equivalent local dipole moment related to the net charges of the atoms of the C --O and HC --O functional groups. Our findings show that the best definition of an equivalent local dipole moment is clearly dependent on the specific features of the charge distribution in the polar region of the molecules (e.g. ketones and aldehydes), which allows them to be distinguished. Thus, the QSRR models for 15 aldehydes and 42 ketones obtained using the remodeled I SET were of good quality as shown by the statistical parameters. The ability of this remodeled index to include charge distribution and structural details opens a new way to study the correlations between the molecular structure and retention indices in gas chromatography.

A new semi-empirical electrotopological index, I SET , for quantitative structure-retention relationships (QSRR) models was developed based on the refinement of the previously published semi-empirical topological index, I ET . We demonstrate that the values of C i fragments that were firstly attributed from the experimental chromatographic retention and theoretical deductions have an excellent relationship with the net atomic charge of the carbon atoms. Thus, the values attributed to the vertices in the hydrogen-suppressed graph of carbon atoms (C i ) are calculated from the correlation of the net atomic charge in each carbon atom, which is obtained from quantum chemical semiempirical calculations, and the C i fragments for primary, secondary, tertiary and quaternary carbon atoms (1.0, 0.9, 0.8 and 0.7, respectively) obtained from the experimental values. This shows that I ET encoded this quantum physical reality and that it is possible to calculate a new I SET (the semi-empirical electrotopological index) through the net atomic charge values obtained from a Mulliken population analysis using the semi-empirical AM1 method and their correlation with the values attributed to the different types of carbon atoms. This demonstrates that the I SET encodes information on the charge distribution of the solute on which dispersive and electrostatic interactions between the solute (alkanes and alkenes) and the stationary phase strongly depend. Thus, this new method can be considered as an initial step towards forthcoming QSRR/QSAR studies.

Gas ch romatog raphy Quantitative structure-retention relationships QSRR Semi-empirical topological method Topological indices Branched alkenes SummaryThe new semi-empirical topological index (lET) previously developed by Heinzen, Soares, and Yunes has been extended to branched alkenes. The main advantage of this method was excellent predictive power and high statistical quality using one topological descriptor only, in comparison with those achieved with a variety of descriptors by multivariate regression techniques and recognition patterns. Another advantage is that this molecular descriptor is related to structural features of the molecule, which enables the cis and trans structures of branched alkenes to be distinguished and highlights the propyl effect -a special steric effect resulting from branching at carbon atoms/? to the dou ble bond. The stabihty and validity of the model generated on squalane (80 ~ was tested by the cross-validation (r2cv = 0.997) technique.