EPR study of a copper center in a single crystal of cytosine monohydrate

A new cytosine-copper paramagnetic complex

“…We have shown that the assignment, here named cytosine-copper complex I, is correct only for the crystals grown from the cytosine aqueous solution, with added small amount of powdered copper. The paramagnetic complex is formed only after ionizing irradiation [4], implying that copper is incorporated in the cytosine matrix as a monovalent cation, probably in a non-paramagnetic Cu-cytosine complex, transforming into a [10]. The possibility could be safely excluded because in that case the plane of the complex would be almost perpendicular to the cytosine rings, which is in contrast to the experimental results.…”

“…Two spectroscopically different paramagnetic copper centers found in the crystal matrix of cytosine monohydrate were assigned to the same structure by two groups of investigators [3,4]. We have shown that the assignment, here named cytosine-copper complex I, is correct only for the crystals grown from the cytosine aqueous solution, with added small amount of powdered copper.…”

“…Cu, having the same spin (I = 3/2) but slightly different magnetic moments [4]. Moreover, two magnetically distinct sites in the crystal give rise to two distinct sets of EPR spectra, making altogether four superimposed quartets, what makes the analysis more difficult.…”

“…The optimization of the structure was followed by single-point calculation of the electronic properties using the same density functional method and 6-311+G(2d,p) basis set. The theoretically derived data are shown in Table 1, together with the two independent sets of experimental values [3,4].…”

“…The data did not fit well the complex of copper with two hydrogen-bonded cytosine molecules (cytosine-copper complex I), although an alternative structure was not feasible. Recently, we showed that indeed, the proposed copper complex (cytosine-copper complex I) with two hydrogen-bonded cytosines gives rise to completely different EPR spectra [4]. That fact stimulated us to further investigate the original system of Toriyama and Iwasaki (cytosine-copper complex II).…”