Benzene-induced solvent shifts of proton resonances in the N.M.R. spectra of α-diketones

Abstract

Solvent shifts [ΔC₆H₆ = δ_CCk4 ? δ_CCl4 p.p.m.] are recorded for a number of acyclic and cyclic α-diketones. Benzene produces upfield shifts of methyl and methylene resonances in the spectra of saturated acyclic and cyclic α-diketones. The shift of the α-methylene resonance of acyclic a-diketones may be related to the length (and bulk) of the side-chain. The formation of a 1 : 1 complex is proposed, and variable temperature studies (in toluene-d_s) have allowed calculation of thermodynamic parameters for an acyclic and a cyclic compound. The dependence of solvent shifts on the position of methyl and hydrogen substituents of aromatic α-diketones has been demonstrated.