Effects of Internal Rotation and Bond Stretching

V(H-C-H) couplings depend considerably on the molecular conformation. All effects were dominated by the changes in the FC contribution. 

Grayson and Sauer computed the coefficients in a Karplus-type equation of the spin-spin coupling constants for a series of rotated ethane geometries. The coupling constants were calculated at the SCF, SOPPA, and SOPPA-CCSD levels and compared with results of previous calculations and experimental data. It was found that the coefficients in the Karplus equation calculated at the SOPPA-CCSD level are in good agreement with coefficients derived from experimental coupling constant data or results of MCSCF calculations. 

---