Electrochemical and density functional theory modeled reduction of enolized 1,3-diketones

Abstract

Cathodic peak potentials ( E pc) of ten enolized 1,3-substituted 1,3-diketones, R 1COCHC(OH)R 2 derivatives containing electron withdrawing and/or electron donating groups, were measured by cyclic voltammetry in acetonitrile. Quantum computational based methods are exploited to model experimentally measured reduction potentials ( E pc) by comparing experimentally measured reduction potentials E pc to the calculated descriptors; LUMO energy ( E LUMO), electron affinity (EA), electrophilicity index (ω) and relative group electronegativity ( χ), obtained from calculated electronic energies of the neutral, anionic and cationic molecules. Observed reduction potentials gave excellent correlation in the linear relationship between experimental E pc and calculated E LUMO ( R 2 > 0.99). Electrochemical behaviour, in agreement with DFT results, show that aromatic β-diketones (containing aromatic side groups) are characterized by reversible CV's due to the stabilization of the radical anion while β-diketones containing aromatic and/or aliphatic groups feature irreversible CV's. The stability of the radical anion is supported by the π-conjugated nature of the LUMO orbitals. The power of the substituent group's inductive effect was determined by using the sum of experimental group electronegativities (Gordy scale) of the R 1 and R 2 groups ( χ R1 + χ R2) and calculated Mulliken electronegativities (in eV). A non-linear relationship between the observed substituent's inductive power and reduction potential ( E pc) was observed since the electron density in the redox centre is controlled by both inductive and resonance effects.