Single-Molecule Magnets Effect of Pressure on the Magnetic Anisotropy in the Single-Molecule Magnet Mn 12 -Acetate: An Inelastic Neutron Scattering Study** Andreas Sieber, Roland Bircher, Oliver Waldmann, Graham Carver, GrØgory Chaboussant, Hannu Mutka, and Hans-Ulrich Güdel* Mn 12 -acetate is the prototype of a class of polynuclear transition metal complexes known as single-molecule mag- nets (SMMs). These spin clusters exhibit new phenomena such as slow relaxation and quantum tunneling of magnet- ization (QTM) at low temperature, [1, 2] and this discovery, about a decade ago, triggered a flurry of interdisciplinary research in physics and chemistry. Mn 12 -acetate was the first SMM discovered, and its properties have been thoroughly studied by many different techniques. As shown in Figure 1, it is composed of a tetrahedral core of oxygen-coordinated Mn 4+ ions, which are surrounded by a ring of eight Mn 3+ ions with oxo and acetate coordination. [3] Dominant antiferro- magnetic interactions between the Mn 4+ and Mn 3+ ions lead to an S = 10 ground state. [4] The Mn 3+ coordination environ- ment is Jahn–Teller-distorted; the elongated Mn O bonds are emphasized in Figure 1. The concerted action of the resulting Mn 3+ single-ion anisotropies leads to an overall easy-axis-type anisotropy of the S = 10 cluster ground state, which can be expressed by Equation (1). ^ H ZFS ¼ D½ ^ S 2 z  1 = 3 SðS þ 1Þ þ B 0 4 ^ O 0 4 where ^ O 0 4 ¼ 35 ^ S 4 z 30 SðS þ 1Þ ^ S 2 z þ 25 ^ S 2 z 6 SðS þ 1Þþ 3S 2 ðS þ 1Þ 2 ð1Þ As a result the S = 10 ground state splits in zero field into eleven  M S sublevels, of which M S =  10 are lowest in energy. An energy barrier between the plus and minus M S sublevels is thus built up (Figure 2). Inelastic neutron scattering (INS), for which DM S =  1 transitions are allowed, is eminently suited for the direct measurement of this splitting pattern in zero field. [5] The physical properties of Mn 12 -acetate can be tuned by chemical variation [6, 7] or by physical perturbations such as an external magnetic field or pres- sure. [8, 9] Here we report the first spectroscopic study of the anisotropy splitting of Mn 12 -acetate under hydrostatic pres- sure up to 12 kbar. From magnetization experiments it was earlier concluded that the anisotropy barrier increases with increasing pressure. [8] From the observed acceleration of the Figure 1. Structure of the core of Mn 12 -acetate (view along the S 4 axis). For clarity only the first coordination sphere of the Mn ions is drawn. The single-ion Jahn–Teller (JT) axes are represented by thick bonds. Mn 4+ : white, Mn 3+ : black, m 3 -O 2 : dark gray, OAc: medium gray, H 2 O: light gray. The two specific sites 1 and 2 are used in the data analysis and discussion. Figure 2. Axial anisotropy splitting and energy barrier D of the S = 10 ground state. The double arrows correspond to the observed DM S =  1 INS transitions. [*] A. Sieber, R. Bircher, O. Waldmann, G. Carver, Prof. H.-U. Güdel Department of Chemistry and Biochemistry University of Bern 3000 Bern 9 (Switzerland) Fax: (+ 41) 31-631-4399 E-mail: hans-ulrich.guedel@iac.unibe.ch G. Chaboussant Laboratoire LØon Brillouin (LLB-CNRS-CEA) CEA Saclay 91191 Gif-sur-Yvette Cedex (France) H. Mutka Institut Laue-Langevin 6 rue Jules Horowitz, BP 156, 38042 Grenoble Cedex 9 (France) [**] This work was financially supported by the Swiss National Science Foundation (NFP 47) and the European Union (TMR Quemolna MRTN-CT-2003-504880). Supporting information for this article is available on the WWW under http://www.angewandte.org or from the author. Angewandte Chemie 4239 Angew. Chem. Int. Ed. 2005, 44, 4239 –4242 DOI: 10.1002/anie.200500171  2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim