title: On the capriciousness of the FCCF Karplus curve.
abstract: 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.
authors.affiliation: Department of Chemistry, Emory University, Atlanta, Georgia 30322, USA.
authors.forename: Serdar
authors.initials: S
authors.lastname: Kurtkaya
authors.forename: Verónica
authors.initials: V
authors.lastname: Barone
authors.forename: Juan E
authors.initials: JE
authors.lastname: Peralta
authors.forename: Rubén H
authors.initials: RH
authors.lastname: Contreras
authors.forename: James P
authors.initials: JP
authors.lastname: Snyder