D. Calculated 1 J C1,C2 in Model Structures 3 c , 4 c and 5 c . DFT calculations on 3 c -5 c were performed as described for 1 c . For 3 c , q 1 was fixed at the angle observed in crystalline 3 (123.86°) and q 2 was rotated in 15° increments through 360°. For 4 c , q 1 was fixed at the angle observed in crystalline 4 1,2 (98.69°) and q 2 was rotated in 15° increments through 360°. For 5 c , q 1 was fixed at the angle observed in crystalline 5 (169.12°) and q 2 was rotated in 15° increments through 360°. The remaining exocyclic torsion angles q 3 -q 6 were allowed to rotate freely during geometry optimization. Calculated 1 J C1,C2 values were plotted as a function of q2