SYSTEMATIC ANALYSIS OF SUBSTITUENT EFFECTS .1. GEMINAL AND VICINAL INTERACTIONS IN FLUOROCHLOROETHANES

The contributions of individual substituents to properties of fluoroch loroethanes are highly nonadditive. The nonadditivies can be accuratel y modeled with sums of terms describing geminal, gauche vicinal, and a nti vicinal interactions between substituents. Such an approach, which requires 12 parameters per property, produces the total and zero-poin t energies and the lengths of the C-C bond with rms errors of 0.8 and 0.04 kcal/mol and 0.002 Angstrom, respectively. The inclusion of all t hree types of interactions is prerequisite to obtaining reasonably acc urate estimates of molecular properties, although in the cases of the total and zero-point energies the geminal contributions strongly domin ate. The present methodology is very general, making it possible to an alyze and predict substituent effects in a systematic manner by summin g transferable increments ascribed to individual substituents, substit uent pairs, triples, etc., until the desired accuracy is attained. The substituent effects in the molecules under study cannot be attributed solely to the phenomenon of electronegativity equalization.