C-13 ISOTOPE EFFECTS IN C-13 NMR-SPECTRA OF SOME CARBONYL SUBSTITUTED BENZENES

C-13 isotope effects on C-13 chemical shifts in proton-decoupled NMR spectra of several C-13-carbonyl substituted benzenes were determined and analyzed. For the first time intrinsic C-13 isotope effects over three bonds from the isotopically substituted site were determined. (n)J(C,C) values for C-13-carbonyl-benzaldehyde 1, C-13-carbonyl-benzoic acid 2, C-13-carbonyl-acetophenone 3 and C-13-carbonyl-benzophenone 4 were determined and their differences interpreted in terms of molecular geometry and conformation. Both C-13 isotope effects and (n)J(C,C) are solvent dependent.