On the capriciousness of the FCCF Karplus curve

Numerous well-behaved Karplus curves correlating dihedral angle (φ) and NMR coupling constants (3JXY) exist for X−C−Z−Y fragments with X or Y = H, C, F, and P. Absent is the enigmatic F−C−C−F moiety. By calculating the four Ramsey contributions to 3JFF for F−CH2−CH2−F, a pleasing φ/3JFF curve with both positive and negative regions arises. Unlike the H−C−C−H curve, F−C−C−F is PSO vs FC dominated. The latter and the F lone electron pairs cause both the negative J values and the substituent-induced J-leveling effect.