A method of natural bond orbital (NBO) interactions between bonds and antibonds, σ m f σ n / , has been developed for analyzing vicinal proton-proton coupling constants, 3 J HH . The contribution to 3 J HH from such an interaction σ m f σ n / is defined as J mn* and is obtained following these three steps: (i) the SOS scheme is used to calculate 3 J HH ; (ii) the same calculation is repeated after deleting the relevant off-diagonal elements in the NBO Fock matrix; (iii) J mn* is obtained as the difference between those calculated in steps i and ii. Application of this method to the ethane and fluoroethane molecules shows that the main contribution to 3 J HH comes from the through space term while the substitution of a hydrogen in ethane by a fluorine changes this through space term and, in addition, gives a direct contribution to 3 J HH .

A model is proposed which assumes that the pseudorotational potential in five-membered rings is given by the combination of contributions from the unsubstituted ring, from the individual substituents and from interactions between pairs of substituents. The application of this model to the potentials calculated by the MM2 force field for the cis and trans-2,5-dimethoxytetrahydrofuran shows that the contributions from the individual substituents explain the main features of the potentials of these disubstituted rings.The pseudorotational analysis from vicinal proton spin-spin coupling constants 3 J~~ confirms the realibility of the MM2 potentials.

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