Investigation on PCET–accompanied Dimerization of 5–hydroxy–1, 4–naphthoquinone in the Process of Electrochemical Reduction by In Situ FT–IR Spectroelectrochemistry and Density Functional Calculation

Electron transfers (ET) of 5–hydroxy–1, 4–naphthoquinone (HNQ) and 5–hydroxy–2–methyl–1, 4–naphthoquinone (HMeNQ) have been investigated in acetonitrile and proton donors mixed media. Although there is merely a small difference in structure between HMeNQ, having a methyl, and HNQ, not having a methyl, the reduction of HNQ involves proton coupled electron transfers (PCET) process while HMeNQ not. Both CV and rapid–scan IR spectra support the existence of intermediate HNQ− and dimer in acetonitrile during electrochemical reduction. Density functional calculations show that proton transfers (PT) occur when forming dimer, indicating that HNQ has PCET–accompanied dimerization mechanism. Besides, dimer also undergoes two successive ET processes to form quinone. When proton donors are added to HNQ, dimerization is inhibited owing to intermolecular hydrogen–bonding, accompanying by the interruption of PCET process.