Hanspeter Andres, Reto Basler, Hans-Ulrich Güdel, Guillem Aromi, George Christou, Herma Büttner and Benoit Rufflé
Inelastic Neutron Scattering and Magnetic Susceptibilities of the Single-Molecule Magnets [Mn₄O₃X(OAc)₃(dbm)₃] (X=Br, Cl, OAc and F): Variation of the Anisotropy along the Series.
J. Am. Chem. Soc. 122, 12469-12477 (2000)                 

Abstract: The single-molecule magnets (SMMs) [Mn₄O₃X(OAc)₃(dbm)₃] (X=Br, Cl, OAc and F) were investigated by a detailed inelastic neutron scattering (INS) study. Up to four magnetic excitations between the zero-field split levels of the lowest S=9/2 cluster ground-state have been resolved. From the determined energy-level diagrams and the relative INS intensities we can show that the inclusion of a rhombic term in the zero-field splitting (ZFS) Hamiltonian is essential in these compounds. On the basis of the Hamiltonian: H_ZFS = D[S_z²-1/3S(S+1)] + E(S_x²-S_y²) + B₄⁰O₄⁰, the following sets of parameters are derived: For X = Cl: D = -0.529cm^–1, |E| = 0.022cm^–1, and B₄⁰ = -6.5e-5cm^–1; for X = Br: D = -0.502cm^–1, |E| = 0.017cm^–1, and B₄⁰ = -5.1e-5cm^–1; for X = OAc: D = -0.4.69cm^–1, |E| = 0.017cm^–1, and B₄⁰ = -7.9e-5cm^–1; and for X = F: D = -0.379cm^–1 and B₄⁰ = -11.1e-5cm^–1. The wave functions derived from the energy analysis are in excellent agreement with the relative intensities of the observed INS transitions. The observed temperature maxima of the out-of-phase component of the variable freqency AC magnetic susceptibility T_max[chi''] correlate well with the energy splittings determined by INS. Direct information about the rate of quantum tunneling is contained in the cluster wave functions derived in this study. The difference in the quantum tunneling between X = Cl and Br is shown to be directly related to differences in the rhombic anisotropy parameter |E|.