Abstract
A study is presented of the structural dependencies for scalar J‐coupling across four bonds in propanic and allylic systems. Density functional theory (DFT) and finite perturbation theory (FPT) were used to obtain the Fermi contact (FC) contributions to ^1^H,^1^H coupling. Interproton couplings ^4^J(H,H′) in propane were obtained as functions of the dihedral angles φ~1~ and φ~3~, which are measured about the C(1)—C(2) and C(2)—C(3) bonds, respectively, and on the C(1)—C(2)—C(3) internal angle θ~2~. A four‐term trigonometric expression reproduces the dependence on φ~1~ and φ~3~ with a standard deviation of 0.1 Hz. The molecular structures for a series of bicycloalkanes were fully optimized and DFT/FPT results for ^4^J(H,H′) are in good agreement with the available experimental data. However, in non‐W arrangements of bicyclic systems the propane trigonometric expression for ^4^J(H,H′) often gives signs opposite to those observed. It is now clear the conformational dependence of coupling constants in propane is not, in general, applicable to bicyclic systems. The DFT/FPT propane‐based results for coupling involving a methyl group (H—C—C—CH~3~) are in better conformity with the experimental data. The DFT/FPT results for cisoid and transoid allylic coupling ^4^J(H,H′) in propene were obtained as functions of the dihedral angle φ measured about the C(2)—C(3) bond. Trigonometric expressions with three terms reproduce the calculated propene results with standard deviations 0.1 Hz. The molecular structures for a series of bicycloalkenes were fully optimized and DFT/FPT results for cisoid and transoid coupling are in fairly good agreement with the available experimental data. In contrast to the propanic coupling, the trigonometric expressions for propenic coupling constants are generally applicable to conformational analyses in bicycloalkenes. Copyright © 2003 John Wiley & Sons, Ltd.