Manifestations of stereoelectronic interactions in 1JC-H one-bond coupling constants.

Two-electron/two-orbital hyperconjugative interactions depend on the relative orientation of bonds and lone pairs in a molecule and are also inversely proportional to the energy difference between the interacting orbitals. Spectroscopic manifestations of stereoelectronic interactions are particularly useful experimental signatures of these effects which can be utilized for testing molecular models. Empirical observations together with theoretical interpretations in cyclohexane and six-membered heterocycles confirm the relevance of sigma C-H ax --> sigma* C-H ax , n X --> sigma* C-H ax (X = O or N), sigma C-S --> sigma* C-H eq , beta-n O --> sigma* C-H eq , sigma C(2)-H ax --> pi* CY (Y = O, S, or CH 2), and sigma C(2)-H ax --> sigma* S-O ax two-electron/two-orbital stereoelectronic interactions that weaken the acceptor (or donor) C-H bonds and attenuate the Fermi contribution to the one-bond (13)C- (1)H coupling constants.