Electron paramagnetic resonance ͑EPR͒ studies of a Ni 4 single-molecule magnet ͑SMM͒ yield the zerofield-splitting ͑ZFS͒ parameters D, B 4 0 , and B 4 4 , based on the giant spin approximation ͑GSA͒ with S =4; B 4 4 is responsible for the magnetization tunneling in this SMM. Experiments on an isostructural Ni-doped Zn 4 crystal establish the Ni II ion ZFS parameters. The fourth-order ZFS parameters in the GSA arise from the interplay between the Heisenberg interaction Jŝ 1 • ŝ 2 and the second-order single-ion anisotropy, giving rise to mixing of higher-lying S 4 states into the S = 4 state. Consequently, J directly influences the ZFS in the ground state, enabling its determination by EPR.

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We report angle-dependent high-field electron-paramagnetic-resonance data collected for single-crystal samples of Mn 12 -Ac. The spectra reveal fine structures associated with various Mn 12 species corresponding to different disordered local environments. Each of the fine structures exhibits a distinct dependence on the field orientation, thereby highlighting the discrete nature of the disorder. We compare these data with the spectra obtained for two recently discovered analogs of Mn 12 -Ac, differing only in their ligand and solvent molecules. None of the fine structures seen for Mn 12 -Ac are found for the recently discovered Mn 12 complexes, thus confirming that the solvent significantly influences the magnetization dynamics in Mn 12 -Ac.

The synthesis and structural, spectroscopic, and magnetic characterization of a Mn25 coordination cluster with a large ground-state spin of S = 51/2 are reported. Reaction of MnCl2 with pyridine-2,6-dimethanol (pdmH2) and NaN3 in MeCN/MeOH gives the mixed valence cluster [Mn25O18(OH)2(N3)12(pdm)6(pdmH)6]Cl2 (1; 6Mn(II), 18Mn(III), Mn(IV)), which has a barrel-like cage structure. Variable temperature direct current (dc) magnetic susceptibility data were collected in the 1.8-300 K temperature range in a 0.1 T field. Variable-temperature and -field magnetization (M) data were collected in the 1.8-4.0 K and 0.1-7 T ranges and fit by matrix diagonalization assuming only the ground state is occupied at these temperatures. The fit parameters were S = 51/2, D = -0.020(2) cm(-1), and g = 1.87(3), where D is the axial zero-field splitting parameter. Alternating current (ac) susceptibility measurements in the 1.8-8.0 K range and a 3.5 G ac field oscillating at frequencies in the 50-1500 Hz range revealed a frequency-dependent out-of-phase (chi(M)'') signal below 3 K, suggesting 1 to be a single-molecule magnet (SMM). This was confirmed by magnetization vs dc field sweeps, which exhibited hysteresis loops but with no clear steps characteristic of resonant quantum tunneling of magnetization (QTM). However, magnetization decay data below 1 K were collected and used to construct an Arrhenius plot, and the fit of the thermally activated region above approximately 0.5 K gave U(eff)/k = 12 K, where U(eff) is the effective relaxation barrier. The g value and the magnitude and sign of the D value were independently confirmed by detailed high-frequency electron paramagnetic resonance (HFEPR) spectroscopy on polycrystalline samples. The combined studies confirm both the high ground-state spin S = 51/2 of complex 1 and that it is a SMM that, in addition, exhibits QTM.