Determination of the intercarbonyl dihedral angle of 1,2-diketones by ¹⁷O nmr

Abstract

This study describes a method using 17 O nmr to determine the intercarbonyl dihedral angle in a homologous series of seven 1,2-diketones. It is shown that the 17 O nmr shifts of the 1,2-diketones are linearly dependent on both the electron density at the oxygen atoms and the n→π* excitation energy of the 1,2-diketo chromophore. Reduction of the overlap in the π system of the 1,2-diketo moiety leads to a gradual upfield shift of the 17 O nmr signal, due to the increase in both the π-electron density at the oxygen atoms [Formula: see text] and the mean transition energy ΔE.