The structural dependence of two-bond 13C?13C coupling constants

Two-bond ¹³C? ¹³C coupling constants are discussed on the basis of INDO-SCPT calculations. The dependence of ²J(CC) on bond angle variation and on methyl substitution is evaluated, and it is shown that ²J(CC) depends linearly on the bond orbital s-character product of the terminal carbon atoms, whereas no systematic relationship with the hybridization of the central carbon atom was obtained. Alkyl group substituent effects are found to be additive. The coupling constants of a number of cyclobutane derivatives are discussed on the basis of these structural relationships; it is shown that the experimental findings can be interpreted quite consistently by assuming a dual-pathway coupling mechanism.