The relationship between structure and electrochemical property of cyanoimino derivatives of squaric acid

Abstract

Theoretical studies on anions of 1,2-dihydroxycyclobuten-3,4-dione (squaric acid, H2SQ) and the whole series of cyanoimino 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 quite strong. The GIAO-DFT results for all dianions suggest that the corresponding B3LYP/6-31+G(d,p) geometries can be deemed reasonably good representations of the geometries of the relatively free dianions in solution. Based on the most stable conformations, linear correlations are observed between the oxidation potential measured by cyclic voltammetry and the highest occupied molecular orbital (HOMO) energy as well as ionization potential (I p), which support experimental results that systematic substitution of the oxygen atoms in the structure with NCN groups causes a shift of both the oxidation potentials and towards more positive values. The correlations are also observed between experimental K sem and Gibbs free energy calculated in AN solution.