Synthesis and characterization of single-molecule magnets: manganese-acetate, iron-bromium, and analogs

This dissertation describes the synthesis and characterization of two new compounds that are among the most studied materials in solid state chemistry over the past eight or so years. The compounds investigated are [Mn12O12(CH3COO)16(H2O) 4]·2CH3COOH·4H2O (Mn12-ac), [(C6H15N3)6Fe8O 2(OH)12]Br7(H2O)Br·8H2 O (Fe8Br8), and several of their analogs. All of these compounds exhibit unusual magnetic ground states, with high electron spin, S = 10, and the novel phenomenon of magnetic quantum tunneling (MQT), whose origin is still not fully understood. We developed methods of growing large, high quality crystals, perhaps the best ones in the literature. We then investigated these materials using electrical conductivity measurements, Raman and infrared spectroscopy, and high-frequency electron paramagnetic resonance (EPR) spectroscopy, and obtained results that should elicit further theoretical and experimental investigations.; Chapter 1 provides a general introduction to the field of study, the topic of single molecule magnets. It summarizes the general properties of these materials and lays the foundation for the studies outlined. Chapter 2 discusses the synthesis of the materials studied and describes the experimental parameters of each of the techniques used for characterization. Chapter 3 reports electrical conductivity measurements on Mn12-ac, and Fe 8Br8, which establish these materials as semiconductors. It also describes measurements that show the photoconductive properties of Mn12-ac. Chapter 4 presents Raman and infrared spectroscopy measurements on both materials. Through the use of model compounds and theoretical calculations we were able to assign the majority of the modes in Mn12-ac and Fe8Br8. These data report the frequencies of several low-lying vibrations that might be important in the mechanism of MQT. The results of high-frequency single crystal EPR measurements of Mn12-ac are reported in Chapter 5. These measurements have shown evidence for a distribution of tilts in the magnetic easy axes of Mn12-ac, which could help to explain the mechanism by which magnetic quantum tunneling occurs in this system. Finally, Chapter 6 reports and summarizes the main conclusions of this dissertation. Taken together, these results should constitute a significant step in our understanding of the unusual properties of these materials, such as MQT, and should also elicit further theoretical and experimental investigations.