Electrochimica Acta, Vol.297, 947-960, 2019
Influence of substituents on the reduction potential and pK(a) values of beta-diketones tautomers: A theoretical study
Insight is provided into the reduction potential, pK(a), energy of deprotonation and other electronic properties of eleven keto-enol tautomers of beta-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 beta-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 pK(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 center dot center dot center dot H hydrogen bonds in the enol forms of the beta-diketones increased for the enolization site closest to the more electron withdrawing side group and proved to be stronger in the beta-diketones with a higher experimental pK(a). The computed hyperpolarizability of beta-diketones was found to be highly dependent on the position of enolization, increasing with a lower band gap, higher polarizabilities (Delta alpha(1), Delta alpha(2)) and lower stability or higher aromaticity, in terms of the exaltation index (Gamma). (C) 2018 Elsevier Ltd. All rights reserved.