EXO-EXO AND ENDO-ENDO VICINAL PROTON SPIN-SPIN COUPLING-CONSTANTS IN NORBORNANE AND NORBORNENE - AN IPPP-CLOPPA ANALYSIS

The IPPP-CLOPPA method is used to study the origin of the difference between exo-exo and endo-endo vicinal interproton couplings in norbornane and norbornene. The main pathway defining such couplings is found to be determined by the corresponding C-H bonds and antibonds. For this reason they can be considered to be transmitted through space. Contributions from excitations involving MOs of bridge 7 and the C-H endo and exo bonds and antibonds are found to be negligible in both compounds. Even though bridges 5-6 and 7 are not included in the coupling pathway, they influence the structure of endo and exo C-H bonds, and thus cause the nonequivalence of endo-endo and exo-exo couplings. Results obtained with the NNBI method are compared to those of IPPP-CLOPPA.