13C NMR and relaxation studies of the nanomagnet Mn12-acetate -: art. no. 064420

The nanomagnet [Mn12O12(CH3COO)(16)(H2O)(4)]. 2CH(3)COOH . 4H(2)O, also known as Mn-12, has been synthesized with C-13 labeling at the CH3 groups, and investigated by C-13 NMR at fields up to 23 T. Using oriented samples, it is possible to resolve four distinct 13C peaks at room temperature, located on both sides of the unshifted Larmor frequency. These peaks were assigned to the four hyperfine-shifted, magnetically inequivalent sets of (CH3)-C-13 groups in the Mn-12 lattice, based on a comparison with the crystal structure and point-dipole and spin-density calculations. These results establish that the unpaired electron spin density of the S = 10 system in this cluster extends over the entire molecular framework, not just the core. These results are discussed in relationship to inelastic neutron scattering measurements. The temperature and field dependence of the C-13 nuclear-spin-lattice-relaxation time T-1 on the least shifted peak was measured. A single weakly field-dependent minimum at about 60 K is observed in the temperature dependence of the measured T-1. The relaxation mechanism responsible for the T-1 minimum is ascribed mainly to hindered rotation of the methyl group of the acetate ligand at higher temperature, and to electronic spin fluctuations at lower temperature.