engleski

Spin couplings in a copper-cytosine complex

In 1975 Toriyama and Iwasaki published the EPR analysis of the Cu(II) interstitially doped in single crystals of cytosine monohydrate. In spite of the fact that they observed only one nitrogen coupling, they proposed the model for Cu(II) to make a planar complex, replacing the double hydrogen bonding between two cytosine molecules. Our EPR measurements reproduce their observations. However, we also prepared a Cu(II)-doped cytosine monohydrate crystal with three nitrogen nuclei interacting with each copper nucleus, as expected for their proposed structure. Our ab initio B3L YP/6- 311+G(2d, p)//B3LYP/6-31G(d) calculations of the cytosine dimer bridged by a Cu(II) ion give the isotropic couplings as follows: a(Cu) = -12.2 mT, a(Nl) = 1.6 mT, a(N2) = 1.8 mT, a(N3) = 2.0 mT. These values are larger than the observed ones: about 20% for the nitrogens and about 50% for copper. Using these results, we proposed scaling for calculated copper coupling parameters that nicely describes the observed parameters. In order to take into account the fact that a possible Cu(II)-complexed dimer is in the crystal lattice, we performed the calculations with fixed coordinates for 3 atoms in each cytosine molecule. The coupling parameters were essentially the same as for a gas complex. In no way we could get only one significant nitrogen coupling. The puzzle still remains unsolved.

spin couplings, EPR, DFT, cytosine monohydrate, copper(II)

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