Unusual magnetic properties of mixed-valence system: Multiconfigurational method theoretical study on Mn-2(+) cation

摘要

The geometry structure, dissociation energy, vibrational frequencies, and low-lying spin-state energy spectrum of Mn-2(+) are investigated by using ab initio CASSCF/ECP10MDF, complete active space self-consistent field/atomic natural orbital basis sets (CASSCF/ANO-s), CASPT2/ECP10MDF, and second-order perturbation theory with CASSCF reference function/atomic natural orbital basis sets (CASPT2/ANO-s) levels of theory. For the ground state the dissociation energy of 1.397 eV calculated at the CASPT2/ANO-s level supports Jarrlod's experimental value of 1.39 eV. The equilibrium bond length and vibrational frequency are 2.940 A calculated at the CASPT2/ANO-s level of theory and 214.4 cm(-1) calculated at the CASSCF/ANO-s level of theory, respectively. On the basis of the mixed-valence model, the Heisenberg exchange constant J(-71.2 cm(-1)) and the double-exchange constant B(647.7 cm(-1)) are extracted explicitly from the low-lying energy spectrum calculated at the higher levels of theory. The magnetic competition between the weaker Heisenberg exchange interactions and the stronger double-exchange interactions makes the ground state a (12)Sigma(g)(+) state, consistent with electron paramagnetic resonance experimental observation, which explains unusual magnetic properties of Mn-2(+), quite different from the antiferromagnetic ground state of Mn-2 and Cr-2. On the other hand, the results calculated at the higher levels of theory show the consistent antiferromagnetic Heisenberg exchange interactions between 3d-3d for Cr-2, Mn-2(+), and Mn-2 (c) 2005 American Institute of Physics.