Hyperconjugative and electrostatic interactions effects on 1J(CH) spin-spin coupling constants (SSCCs) are critically studied from both theoretical and experimental points of view. A qualitative model is used to predict how the former affect such SSCCs, while electrostatic interactions are modeled with a point charge placed in the vicinity of the corresponding sigma(CH) bond. Hyperconjugative interactions are calculated using the "natural bond orbital" approach, and using the point-charge model, it is shown how intertwined are both types of interactions. Several members of the series 1-X-bicyclo[1.1.1]pentane and 1-X-3-methylbicyclo[1.1.1]pentane are chosen as model compounds for measuring 1J(CH) SSCCs; in some of them were performed also DFT-SSCC calculations. The strained cage substrate in these series defines strong sigma-hyperconjugative interactions, making these compounds excellent examples to verify the qualitative model presented in this work. It is verified that (a) hyperconjugative interactions from the sigma(CH) bond or into the sigma(CH) antibond containing the coupling nuclei yield a decrease of the corresponding 1J(CH) SSCC and (b) hyperconjugative interactions from other bonds involving the coupling C nucleus yield an increase of that 1J(CH) SSCC.

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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 .