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.

SummaryTheoretical prediction of chromatographic retention couJd be used as a means of identification of organic compounds separated bygas chromatography.An optimum semi-empirical topological index, JET(opt), is proposed for prediction of the chromatographic retention of the methylbranched alkanes produced by insects. Our previously-developed semi-empirical topological index (lET) has been improved to make it possible to distinguish bel',,veen methylalkane isomers. The new topological index is based on the length of the chain, the positions of the methyl groups, and the number of methyl groups attached to the backbone. Simple linear regression belvveen chromatographic retention and the proposed optimum semi-empirical topological indexes, for 178 methylalkanes with nine to forty carbon atoms in the linear chain, was very satisfactory (r 2 = 0.9999 and SD = 3.20). The good predictive quality of the model is shown by the chromatographic retention indexes predicted for an external group of 30 methylalkanes.