Carbon-13 Isotope Effects in ¹³C NMR Spectra of Some Carbonyl Substituted Benzenes

Abstract

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

Key Words:

• ¹³C NMR, ¹³C isotope shifts, ¹³C-¹³C coupling constants, ¹³C-carbonyl labeled species