Journal of Physical Chemistry A vol:113 issue:21 pages:6149-6157
The Structural, electronic and magnetic properties of two different models of the heterospin polymer chain complexes of Cu2+ hexafluoroacetylacetonate with two pyrazole-substituted nitronyl nitroxides Cu(hfac)(2)L-R have been studied by means Of multiconfigurational perturbation theory based oil a CASSCF (complete active space self-consistent field) wave function, i.e. the CASPT2 method. Our calculations reveal the presence of two minima in the electronic energy curve along the Cu-O-L bond, separated by only 6 kcal/mol, and corresponding to the X-ray structures of the CuO6 centers in Cu(hfac)(2)L-Pr at 115 and 293 K, respectively. The two energetic minima are characterized by a different electronic structure, thus giving rise to a different three-spin exchange coupling and explaining the thermally Induced spin transitions in this family of compounds. The concomitant variations ill the magnetic properties, i.e. g factors and magnetic moments mu(eff)(T) were calculated and compared with the experimental data Of Cu(hfac)(2)L-Pr. Even if the correspondence is only qualitative, our Calculations provide a convincing explanation of the observed magnetic peculiarities. In particular, at low temperatures, the predicted ground-state is (2)A(u), well separated from the 2A(g), (4)A(u) states and therefore exclusively populated. Its calculated g factors, g(parallel to) = 1.848, g(perpendicular to) = 1.965, 1.974, qualitatively correspond to the observed g < 2 signals ill the low-temperature EPR spectra. The previously assumed formal spin assignment >N-O-center dot-Cu-O-center dot-N< for these linear spin triads is challenged by our calculations, pointing instead to a more important role of the end-standing NO ill the exchange interactions with Cu(II).