On the capriciousness of the FCCF Karplus curve.
Numerous well-behaved Karplus curves correlating dihedral angle (phi) 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 phi/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.
Kurtkaya
Serdar
S
Department of Chemistry, Emory University, Atlanta, Georgia 30322, USA.
Barone
Verónica
V
Peralta
Juan E
JE
Contreras
Rubén H
RH
Snyder
James P
JP