Insights into the DMH tautomeric structures and its effects on the electro-reduction of Au(DMH)4− coordination ions

5,5-Dimethylhydantoin (DMH), as a heterocyclic compound with N and O atoms, is an excellent complexing agent for trivalent gold ions . In this work, the tautomeric structures of DMH at different pH values in the solution are determined via theoretical calculations and Raman spectra . Only the ketonic form of DMH exists in weakly acidic and neutral mediums, and only the enolic form of DMH exists in highly alkaline mediums. In the weakly alkaline medium of pH 9, a large number of ketonic forms and a small number of enolic forms coexist. The coordination behavior between DMH and Au(Ⅲ) is clarified in the weakly alkaline medium of pH 9. The DMH in ketonic form is fully coordinated with Au(Ⅲ) at pH 9 and the optimal coordination structure is N2-Au with an N-Au bond energy of 16.17 eV. The electrochemical study reveals that Au(DMH) 4 − coordination ions are in the two-step electro-reductions with the first Au(Ⅲ) to Au(Ⅰ) and the second Au(Ⅰ) to Au on the gold electrode surface. The EC-SERS experiments further verify the interfacial process of tautomeric structures and Au(DMH) 4 − coordination ions.