A molecular orbital study of the dihedral angle dependencies of vicinal carbon-carbon coupling constants

Abstract Theoretical studies are presented for the conformational dependencies of vicinal 13 C 13 C coupling constants within a variety of saturated and unsaturated molecular frameworks. Using the MO approach of Blizzard and Santry ( J. Chem. Phys. , 55 , 950 (1971); 58 , 4714 (1973)) , it is shown that three distinct patterns of dihedral angle vs 3 J CC contact coupling are produced, depending upon the degree of double bonding present within the direct coupling path. By means of calculations that involve modifying the various exchange integrals, two of these coupling patterns are found to depend on δ-π exchange. A rationalization of the origin of these coupling patterns is presented. In all systems studied, the orbital and dipolar contributions to the vicinal coupling constant are calculated to be negligible compared to the Fermi contact mechanism except in conjugated systems; in these cases the magnitude of the dipolar term is highly dependent on the dihedral angle and is approximately one-third of the total 3 J CC at dihedral angles of 0 and 180°. The effect of substituents on 3 J CC is discussed.