Excursions in Molecular Simulations and Thermodynamics of Lanthanides with Crown Ethers

Owing to the onset of applications of technological devices in our everyday life, lanthanide elements are becoming continuously important. Traditionally known as rare earth metals, lanthanides (Ln) are categorized as critical materials by the technological companies. Their broad utilizations comprise catalysis, permanent magnetics, phosphorus, etc. Ln help creating clean energy by being employed in electric vehicles and technological devices like smartphones. The massive usages of Ln in industry, however, could have a drastic impact on the sustainable use of these elements. Thus, one needs to address this issue by studying the lanthanide separation where we can recover them and provide more resources for their re-use. This viral use of employment of lanthanides in high-tech economies and the limited understanding of their coordination environment will present new opportunities for lanthanide recycling from waste separation techniques. Historically, trivalent lanthanides were thought to be the only accessible oxidation state of these element, however, after successful synthesis of the divalent species nearly a decade ago, they start to attract lanthanide chemists. Becoming a new emerging subfield of lanthanides, divalent lanthanides (Ln2+) are still lacking many experimental and theoretical aspects of which thermodynamics properties are very important and make the experimental studies challenging since Ln2+ are very unstable species. This complexity introduces new opportunities to engage theoretical studies to uncover the complex behavior of these newly growing subfield of Ln. Chapter 1 will provide background information on the lanthanides, as well as their importance, applications, and the need for recycling them. Information regarding the need and challenges for theoretical studies of these elements is discussed. Chapter 2 builds a theoretical foundation for studying the divalent lanthanides theoretically. Given the dynamics aspects of Ln2+ are of important and interest, potentials were developed for AMOEBA force field which is necessary to use in molecular dynamics (MD) simulation for solving Newton's equation of motion. Since water is of great importance from many aspects, hydration of the two most stable Ln2+, Sm2+ and Eu2+, as well as their coordination environment and phenomena affecting the coordination like water exchange mechanism is studied and analyzed in depth. Details about the coordination environment of high concentration of Sm/EuCl2 salts are provided, too. After paving the path for theoretical studies of Ln2+, chapter 3 lays the ground for studying the complexation of several Ln2+, Sm2+, Eu2+, Dy2+ and Yb2+, and one divalent actinide, Cf2+ and their halide (Cl−, Br−, I−) salts with dicyclohexano-18-crown-6 (DCH18C6) in two different solvents, tetrahydrofurane (THF) and acetonitrile. DCH18C6 is a promising crown ether that is used in an industrial scale in lanthanide separation. This crown ether has five common diastereoisomers of which cis−syn−cis, cis−anti−cis are the best complexants. Being able to form a bowl shape, the trans − anti − trans is also added to our study to observe if encapsulating the cations would lead to a better complexation compare with the mentioned diastereoisomers. It is worth mentioning that the latter diastereoisomer is not well studied like the other two. As such, the first step is to parameterize the mentioned new cations plus THF and DCH18C6 for AMOEBA force field. In the next step, MD simulations were carried to see the conformational changes of all diastereoisomers in the solvents. In addition, the Ln trend for both acids and bases were studied with Dy, Yb and Cf. Following the MD simulation, to gain better insight into the selectivity of these diastereosiomers, binding free energy calculations were performed with Bennet's acceptance ratio (BAR) method for SmBr2 systems in the absence of experimental values. Chapter 4 demonstrates the complexation of Sm2+ and Eu2+ and their halide (Cl−, Br−, I−) salts with a larger crown ether, dibenzo-30-crown-10 (DB30C10). This crown ether shows larger conformational fluctuation because of having a larger polyether ring. As such, conformational analysis from both energy and structure points of view has been done. Moreover, for comparing the complexation of this crown ether with the one mentioned in chapter 3, binding free energy of SmBr2 is calculated and reported without a direct comparison with experimental value. Finally, chapter 5 will provide the concluding remarks by comparing the obtained results.