High Spin Mn Molecular Clusters: Spin State Effects on the Outer Core-Level Multiplet Structures
- PDF / 205,968 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 29 Downloads / 206 Views
HIGH SPIN Mn MOLECULAR CLUSTERS: SPIN STATE EFFECTS ON THE OUTER CORE-LEVEL MULTIPLET STRUCTURES A. J. NELSON*, J. G. REYNOLDS* and GEORGE CHRISTOU** * Lawrence Livermore National Laboratory, Livermore, CA 94550 ** Indiana University, Department of Chemistry, Bloomington, IN 47405 ABSTRACT Oxo-bridged manganese polynuclear complexes have applications in a variety of technologies, such as single-molecule nanomagnets, catalysis and photosynthetic redox chemistry. The reason that these types of compounds are capable of such important and varied technologies is thought to be because they possess ground states with large spin values. However, the electronic, structural and magnetochemical relationships are not well understood and need to be thoroughly investigated to adequately explain why Mn is such an integral part of so many useful processes. X-ray photoemission spectroscopy was used to study the Mn 3p, 3s and valence band electronic behavior as a function of Mn cluster structural properties, where the cluster size and nuclearity are systematically varied. Results show a chemical shift of the Mn 3p3/2,1/2 spin-orbit pair related to the cluster size and nuclearity. Also, the Mn 3s 7S and 5S final state multiplet components shift since they involve the binding energy of a ligand valence electron. In addition, the branching ratio of the 7S:5S states is related to the 3s–3d electron correlation. Specifically, in the 7S state, the remaining 3s electron is well correlated with 3d electrons of parallel spin, while in the 5S state the two spins are antiparallel. Changes in this electron correlation are clearly observed in the 7S:5S branching ratio as a function of cluster size and ligand electronegativity. INTRODUCTION The multiple oxidation states of transition metal ions make them ideally suited for multielectron processes. Clusters of transition metal ions have been found at the active sites of numerous electron transfer enzymes, notably the tetramanganese cluster in the oxygen evolving complex, the site of water oxidation in photosynthetic organisms.[1] Also, Mn=O, where the oxidation state of Mn is +4 or +5, has been identified as the reactive species in catalytic oxidations with Mn porphyrins.[2,3] These catalytic reactions usually end with the complete conversion of the Mn to permanganate, Mn(VII).[4] Oxo-bridged manganese polynuclear complexes have also proved useful in the development of singlemolecule nanomagnets. [5,6] These molecules have a large ground state spin, S, and a large magnetic hysteresis comparable to that observed in hard magnets. This provides the possibility of molecular bistability, opening the way to store information at the molecular level. Spin state effects can be examined by x-ray photoemission spectroscopy of outer core-level multiplet structures. Previous photoemission studies on transition metal compounds reveal core-level multiplet structures that are best understood in terms of configuration-interaction (CI) calculations including intrashell electron correlation, charge-transfer and final
Data Loading...
