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Density functional theory investigation of the proton hyperfine couplings in thiocytosine oxidation product in the cytosine monohydrate crystal lattice

The spin density distribution and the proton coupling tensors of the neutral thiocytosine electron-loss radical in the crystal matrix of cytosine monohydrate were studied with the use of the density functional theory (DFT) at the B3LYP level. It is shown that the isolated radical approximation makes a poor model for the explanation of all the features observed with the ENDOR spectroscopy. More complex structures, that include one or more neighboring cytosine molecules of the lattice, make more representative systems. The calculations of the geometry optimization with the use of the 6-31G(d) basis set and the single-point calculations with the 6-311+G(2d, p) basis set give the spin distribution and the proton coupling tensors in good accord with the observed values. The calculations confirm the H1 proton transfer from thiocytosine to N3 of the neighboring cytosine molecule along the hydrogen bond. This work represents the first complete DFT analysis of the distant, intermolecular proton coupling tensors for a radical in solids.

DFT calculations; thiocytosine cation radical; proton couplings; spin density distribution

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