A theoretical study of electrical and electrochemical properties of dicyanomethylene derivatives of squaric acid

Theoretical studies on anions of 1,2-dihydroxycyclobuten-3,4-dione (squaric acid, H2SQ) and the whole series of dicyanomethylene derivatives in gas-phase and in acetonitrile (AN) solution are carried out using density functional theory (DFT) and SCRF-DFT method at B3LYP theory level for the first time. Natural bond orbital (NBO) analyses indicate that π-electron delocalization in the series is stronger. Based on the most stable conformations, a linear correlation was observed between the highest occupied molecular orbital (HOMO) energies, or ionization potential (Ip) and the oxidation potentials measured by cyclic voltammetry, which supports experimental results that systematic substitution of the oxygen atoms in the C4O42− structure with C(CN)2 groups causes a shift of both the oxidation potentials E10 and E20 towards more positive values, indicating that a consistent delocalization of the negative charge is exerted by these substituents and monoanions are more resistant against oxidation than their counterparts-dianions. The correlations were also observed between experimental Ksem and Gibbs free energy calculated in AN solution. The calculated gas-phase relative basicity of the six dianions follows the order SQ2− > 4-CDCB2− > 2,4-CDCB2− > 3,4-CDCB2− > 2,3,4-CDCB2− > 1,2,3,4-CDCB2− which support the experimental results that the dianions SQ2− and 4-CDCB2− can be protonated, in strongly acidic medium, to the corresponding monoanions, whereas no protonation occurs in the case of di-, tri- and tetra-substituted compounds.