The estimation of the on-site coulomb repulsion energy of 2,2′-bi-1H-imidazole system

The 2,2′-bi-1H-imidazole (H2BIM⁰) can change to 2,2′-bi-1H-imidazolium (H4BIM²⁺) by the two-step proton-transfer, then, to 2,2′-bi(2,3-dihydroimidazolylidene) (H4BIM⁰) by the two-step electron-transfer. From the systematic investigations of half-wave redox potentials (E _1/2) and acid dissociation constants (pK_a), we estimated the repulsion energy of electrons (U₁^CT) and protons (U₁^PT) for H4BIM⁰ and H4BIM²⁺, respectively, and other systems, p-benzoquinones (BQs), diphenoquinones (DPQs), 7,7,8,8-tetracyanoquinodimethanes (TCNQs), tetrathiafulvalenes (TTFs), aromaticdiamines, hydroquinones (H2Qs), 1,4-benzenedimalononitrile (H2TCNQ), bis(4-hydroxyphenyl) sulfides (HPSs). The U₁^CT and U₁^PT values are deduced from the linear correlation between the first E_1/2¹ (pK_al) value and the second E_1/2²(pK_a2) one. The obtained U₁^CT of H4BIM⁰ (4.68 eV) is comparable to that of TTF (4.65 eV), and U₁^PT of H4BIM²⁺ (4.26 eV) is slightly larger than that of H2TCNQ (4.22 eV). The U₁^CT and U₁^PT values are correlated to the structural parameter of φ-electron density and molecular length.