Determination of the group electronegativity of CF3 group in 3 '-O-CF3-thymidine by H-1-NMR

The interplay of enthalpy of the gauche effect (Delta H degrees(GE)) Of the [X3'-C3'-C4'-O4'] fragment in various 3'-substituted (X) 2',3'-dideoxythym idine derivatives 1 - 7 and the inherent anomeric effect drives the two-sta te North reversible arrow South equilibrium in the constituent sugar moiety . The group electronegativity of 3'-OCF3 substituent in Marriott's, Inamoto 's and Mullay's scales has been determined from simple calibration graphs c orrelating the group electronegativity of various 3'-substituents (X) in 2' ,3'-dideoxythymidine derivatives 1 - 7 with the experimental strength (Delt a H degrees(GE)) Of the [X3'-C3'-C4'-O4'] gauche effect. Delta H degrees(GE ) has been experimentally determined from pseudorotationaI analyses of temp erature-dependent (3)J(HH) coupling constants, and can be used as an unambi guous tool for direct experimental estimation of the group electronegativit y of a specific substituent covalently attached to 3'-carbon of 2',3'-dideo xythymidine, which can be compared, in turn, with the theoretical estimatio n carried out according to Marriott's or Inamotols procedure. Inconsistency found between theoretical values in Marriott's and Inamoto's scales, on th e one hand, and between our experimental estimate and the theoretical value in Marriott's scale, on the other, have been solved by refining the electr onegativity scale using our experimental data for 1 - 7.