Study On The Crystallization Modulation Capability Of Rare Earth Ions In Aqueous Solution System

Rare earth?RE?elements possess excellent light,electricity,magnetism and heat properties for their particular atomic structure and abundant electronic energy levels,which promote RE elements wide range of applications in industry and high-technology fields.Whether 4f orbitals of lanthanide elements can participate in chemical bonding is still a controversial topic.The one hand considers the localized 4f orbitals that are shielded from perturbation by 5d and 6s shells for lanthanide elements.The spatial extent of 4f orbitals is small,limiting the overlap with ligands orbitals and strong localized property,resulting in the core-like 4f orbitals that do not involved in chemical bonding.On the other hand,the radial distribution of 4f orbitals has a small overlap with 5d orbitals and 4f orbitals are sensitive to their surrounded chemical environment.So,4f orbitals can participate in chemical bonding.However,whether 4f orbitals of lanthanide elements can participate in chemical bonding related to the strength of coordination field.Weaked ligands probably induce the participation of 4f orbitals in chemical bonding.The 4f orbitals act as a part of core and set up a series of parameters carrying out theoretical research on RE element chemical bonding.The coordination number of RE atoms have 8-or 9-coordinated in RE elements complexes with a low coordination number,and 4f orbitals do not involved in chemical bonding.Xue et al.have demonstrated that 4f orbitals also have the probability to participate in chemical bonding via hybridization with the other sub-shells in RE atoms.Specially,it can be found that the coordination number of RE atoms should be larger than 9.The chemical bonds and molecule geometry of RE complexes can be directly affected by the participation of 4f orbitals in chemical bonding.Therefore,the properties of chemical bonding between the central ion and ligands are determined by valence shell hybride orbitals and ligands property.In-situ Raman and attenuated total reflectance-infrared?ATR-IR?spectroscopy are carried out to study on the evolution of aggregates and structure of RE complexes during crystallization process at molecule level.We can deduce the structure of RE complexes on the basis of evolution of chemical bonding strength and molecular symmetry.Futhermore,based on the coordination number of central ions,we speculate whether the 4f orbites are involved in bonding and clarify the internal relationship between the bonded 4f orbitals and the geometric configuration of RE complexes.Based on the above problems,this paper carried out the following work:?1?During the nucleation process in urea aqueous solution,rigid urea molecules are linked by hydrogen bonding without any changes in molecular symmetry,which makes it difficult to track the nucleation by identifying the symmetric variation of urea in a solution system.In this work,combining in situ Raman and infrared spectroscopy was found to be powerful to observe the fine variations of different groups such as C=O,CN,NH₂,and OH in urea molecules at the nucleation stage.The dehydration of the hydrated urea and the aggregation between urea molecules were experimentally confirmed in the nucleation.According to the evolution of vibration bands of these typical groups,three states can be clearly distinguished,i.e.,hydrated monomers,prenucleation clusters,and crystalline nuclei.In the initial period of nucleation,hydrated urea monomers and clusters coexist in the solution.With an increasing the size of urea aggregations,prenucleation clusters are formed,which then transform into crystalline nuclei within a very short time.During this period,a rapid structural adjustment occurs,which can be identified by the dramatic variations of both wavenumbers and vibration intensity of constituent groups in urea molecules.Finally,crystalline urea nuclei are formed and grow,during which the recorded Raman and ATR-IR signals remain unchanged.Our present work will deepen the understanding of the nucleation of rigid molecules nearly without symmetric variations in a solution system.?2?Rare earth ions can be used to construct a variety of novel structures and are favorable to chemical bonding regulation and design.In this study,the chemical bonding paradigm between rare earth ions(Ln³⁺)and urea molecules in an aqueous solution can be tracked by the evolution of C=O,NH₂,and CN vibration bands during the urea nucleation stage.Rare earth ions such as La³⁺,Gd³⁺,and Lu³⁺can manipulate the nucleation time of urea via regulating the nucleation-dependant N–C=O…H–N hydrogen-bonding between urea molecules.Two types of chemical bondings between Ln³⁺and urea molecules have been confirmed,which are Ln³⁺…O=C–N and Ln³⁺…NH₂-C.Compared with Ln³⁺…NH₂-C,Ln³⁺prefers to coordinate with the O=C bond in urea.With a higher concentration of rare earth ions in the solution,some N–C=O…H–N hydrogen bonds are broken as a consequence of the incorporation of Ln³⁺into the lattice,resulting in the decreased symmetry of local urea molecules in the crystalline nuclei and the consequent Ln³⁺concentration-dependent nucleation time of urea.Moreover,using the ionic electronegativity scale of Ln³⁺,the different effects of La³⁺,Gd³⁺,and Lu³⁺on urea nucleation can be further distinguished.The present study provides basic data for unrevealing the chemical bonding regulation role of rare earth ions in the formation of hydrogen bonded materials,which may give insight into the design and fabrication of novel materials utilizing rare earth ions to adjust the chemical bonding process.?3?The chemical bonding of lanthanide elements serves as one of most important topic in 4f chemistry,however,whether or not 4f orbitals participate in chemical bonding is still debated.One of direct performance of the chemical bonding between lanthanide atom and ligands is the coordination number,depending on the orbital hybridization of central cation and the coordinate ability of ligands.Herein we evaluate the orbital hybridization of Ce^IV by tracking the variations of Ce^?-O bonding in the ligand exchange reaction.The substitution of ligand would induce different outer orbitals of Ce^? participating in the chemical bonding,which can directly influence the coordination number and molecular geometry.By tracking the structural evolution of Ce^? complex using in situ Raman spectroscopy,we deduce that the participation of 4f orbitals in hybridization will increase the coordination number of Ce^?,weaken the Ce^?-O bonding,and offers great space conveniences for geometry construction.The present work demonstrates the possibility of spectroscopy identification of modes of orbital hybridization and participation of 4f orbitals in chemical bonding.