The stereochemical significance of the long-range proton spin coupling constant of the σ-fragment HCCCH

Abstract

In the conformational analysis of organic compounds by proton magnetic resonance of fundamental importance is the dependence of the value of the proton-proton spin-spin coupling constant on the geometrical arrangement of the corresponding hydrogen atoms. This relationship has been quite thoroughly investigated for geminal and vicinal protons and is being extensively employed in conformational studies. On the other hand the no less important relations of the long-range proton spin coupling constant ( 4 J) through four sigma bonds (HCCCH) have been much less systematically investigated. In the present work the magnitudes and signs of 4 J have been determined in substituted tetrachlorobicyclo[2,2,1] heptanes, whose structural framework possesses sufficiently fixed angular parameters. The results from these model compounds have served as basis for an empirical correlation between 4 J and the dihedral angles of the fragment HCCCH, useful for structural purposes, and have also permitted an estimation of the contributions to the value of 4 J made by direct and indirect coupling of the protons.