X-ray photoelectron spectrometry of copper-thiouracil complexes

X-ray photoelectron spectrometry was applied to evaluate the correct oxidation number of copper in complexes with 2-thiouracil, 6-amino-2-thiouracil and 6-methyl-2-thiouracil. Regardless of the mode of preparation exclusively Cu(I) was bound to the different thiouracil compounds, producing one homogeneous signal of the Cu2p3/2 electrons at 932.5 eV. Upon oxidation with H2O2, a typical Cu(II) satellite appeared in the main signal of the Cu2p3/2 level was shifted to higher binding energy values. The reaction of Cu(II) with thioracil yielded identical complexes as above, in which Cu had the formal oxidation state +I. During this reaction portions of the thiouracil were oxidized to 2,2'-dithiobis(4-pyrimidinol) [bis(4-hydroxy-2-pyrimidinyl)disulphide], as seen by the shifted sulphur 2p signals to 163.5 eV. After treating the Cu-thiouracil complexes with H2O2, the simultaneous presence of sulphur species having the oxidation states RStheta (161 eV), RSSR (163.5 eV) and RSOtheta3 (168.5 eV) is indicated by the ESCA signals monitored.