Chemical Meaning of Octane Number Analyzed by Topological Indices*

Correlation of the octane number (ON) of heptane and octane isomers with various topological indices, Z, w, p, and B, was studied. For single parameter correlation the index B by Balaban and w by Wiener show good results, whereas for two-parameter correlation almost any combination among the above four indices was found to give relatively high r value. Further, by adding the third parameter the correlation is a little improved. One of the best results of regression analysis is: ON = 76.389 + 8.179 (0.3B + 1.1p – 0.6Z) with r = 0.982 and S = 6.56 for octane isomers. From these analyses one can conclude that in order for a gasoline isomer to have high ON, or to be burnt without knocking, it should be highly branched (small Z or low boiling point), its statistically averaged shape should be spherical (large B), and it should have a compact structure (large p or high liquid density). The weight of the contributions of these factors decreases in this order. From these results several candidates of nonane isomers whose ON is expected to be higher than 100 are suggested.