Influence of substituents on the reduction potential and pKa values of β-diketones tautomers: A theoretical study. (20th February 2019)

• Record Type: Journal Article
• Authors: Adeniyi, Adebayo A.; Conradie, Jeanet
• Is Part Of: Electrochimica acta. Volume 297(2019)
• Page Start: 947
• Page End: 960
• Publication Date: 2019-02-20
• Subjects: Reduction potential -- pKa -- Deprotonation -- QTAIM -- Hyperpolarizability -- Polarizability -- NEDA; Electrochemistry -- Periodicals; Electrochemistry, Industrial -- Periodicals; 541.37
• DOI: 10.1016/j.electacta.2018.12.030
• Languages: English
• ISSNs: 0013-4686

Abstract

Abstract: Insight is provided into the reduction potential, p K a, energy of deprotonation and other electronic properties of eleven keto-enol tautomers of β-diketone derivatives, using density functional methods (DFT) and higher computational G3(MP2) methods. The computed reduction potential significantly reproduced the experimentally reported reduction potential values, with a mean absolute deviation (MAD) of 0.037 eV when using the higher computational G3(MP2) method, and a MAD of 0.061 eV when using the M06/6-311+G(df, p) DFT only method. Calculations proved a greater ease of reduction for the enol form of the β-diketones, indicated by higher (less negative) reduction potentials than their keto counterparts. The enol forms showed a further differentiation, with even higher reduction potentials observed when enolization occurred furthest from the more electron withdrawing side group. Latter enols also resulted in a lower dipole of the linking fragment between the electron donor and acceptor fragments (side groups). The computational results further provided more insight into the experimentally observed p K a values, suggesting the possibility of proceeding by deprotonation of the hydroxy group from the enol form of the molecules, rather than their keto form. The strength of the O⋯H hydrogen bonds in the enol forms of the β-diketones increased for the enolization site closest to the more electron withdrawing side group and proved to be stronger in the β-diketones with a higher experimental p K a . The computed hyperpolarizability of β-diketones was found to be highly dependent on the position of enolization, increasing with a lower band gap, higher polarizabilities (Δα1, Δα2 ) and lower stability or higher aromaticity, in terms of the exaltation index (Γ).