Dresden 2006 – wissenschaftliches Programm

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MA: Magnetismus

MA 37: Molecular Magnetism

MA 37.3: Vortrag

Freitag, 31. März 2006, 11:15–11:30, HSZ 403

A ²³Na-NMR study of the ferric wheel system Na@Fe₆ (tea)₆ — •Lars Schnelzer, Roland Leppin, and Bernd Pilawa — Physikalisches Institut, Universität Karlsruhe (TH), D-76131 Karlsruhe

The hexanuclear iron(III) complex Na@Fe₆(tea)₆ consists of a ring of six Fe(III) ions which are coupled by organic ligands and centred by a ²³Na-alkali atom. The dynamic properties of Na@Fe₆(tea)₆ have been studied by ¹H and ²³Na NMR measurements at  52 MHz. The temperature dependence of the ¹H T₁ rate reveals a peak at  30 K which is a characteristic feature of the ferric wheel systems whereas the ²³Na NMR T₁ rate measurements increases linearly with temperature indicating a purely quadrupolar relaxation of the nuclear spin. In order to study the influence of the electronic spin system on the NMR properties of the ²³Na nucleus, the magnetic field and temperature dependence of the T₁ rate has been measured on a polycrystalline sample. The field dependence shows the strong enhancement of the T₁ rate due to the level crossing between the S=0 groundstate and the first excited S=1 state in the field range between B_∥ ∼11 T and B_⊥ ∼13 T, proving the influence of the Fe(III) spin on the ²³Na nucleus. In contrast to the measurements at 52 MHz, the temperature dependence of T₁ at 79.5 MHz is characterized by a strong increase up to T ∼30 K and a broad maximum, resembling the ¹H measurements of T₁⁻¹ on the Fe₆(tea)₆ system, when the magnetic field is oriented parallel to the molecular symmetry axis.