| The focus of this dissertation was to carry out an in-depth investigation by various spectroscopic techniques of the single-molecule magnets (SMM's) Mn12-acetate ([Mn12O12(CH3COO) 16(H2O)4].2CH3COOH.4H2O) and Fe8Br (Fe8O2(C6H15N 3)6.Br8.H2O. Dielectric measurements were made at frequencies of 20 Hz-1 MHz over the temperature range of 2.8--300 K on Mn12-acetate, using normal, partially deuterated, and fully deuterated samples. The results indicate the presence of several relaxation modes at T > 30 K. The three slowest modes could be ascribed to the motion of lattice H2O, while the fourth one, labeled M4, could be assigned to motion of CH3 groups. The relaxation behavior of M4 was found to be essentially similar to that of CD3 motion. EPR measurements were carried out over a wide range of frequencies (10--200 GHz), using single crystals of both Mn12-acetate and Fe8Br. These data yielded accurate values for the parameters of the S = 10 Hamiltonian. Another significant question investigated herein is how well the spin systems are described by the giant-spin, S = 10 model, specifically, whether the unpaired electrons reside only on the metal ion framework, or are also distributed to some extent over the organic ligands. This was accomplished through 13C NMR measurements on specifically labeled samples of Mn12 -acetate. Large, temperature-dependent contact shifts were observed for the 13C nuclei at both the -COO and -CH3 moieties. Angular variation studies yielded the Fermi-contact part of the 13 C hyperfine coupling, from which the unpaired electron spin density was determined on each carbon site. The data indicate about 10% spin density on these sites, helping to explain discrepancies between neutron scattering and other techniques. |
