Spectroscopic studies in olefins. VI. The valence angle dependence of interproton nuclear magnetic resonance coupling constants

Abstract

INDO–MO calculations have been performed for all the proton–proton coupling constants in ethylene, propylene, and cis- and trans-2-butene, both at their equilibrium geometry and at geometries deviating from equilibrium, thus resulting in [Formula: see text] parameters for all coupling constants as a function of all valence angles[Formula: see text]. It is shown that such valence angle effects can be appreciable, particularly for two- and three-bond couplings. The valence angle effects are demonstrated to be independent and additive. The calculated data, in combination with the exact geometrical data and experimental values for the coupling constants allow the separation of (inductive) substitution and rehybridization effects. It is also shown that for three-bond couplings the effects of in-path C—C—H bond angle changes are minor in comparison to the exo-path C=C—C valence angle effects. The new data provide a consistently good model to explain certain empirical trends in two- and three-bond coupling constants as a function of the size of the substituents, assuming a simple steric hindrance induced rehybridization mechanism. On the other hand it is also found that such a mechanism cannot account for similar trends in the H—C=C—CH3 allylic and the CH3—C=C—CH3 homoallylic coupling constants.