Nucleation-dependant chemical bonding paradigm: the effect of rare earth ions on the nucleation of urea in aqueous solution

Abstract

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 CO, 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–CO⋯H–N hydrogen-bonding between urea molecules. Two types of chemical bondings between Ln³⁺ and urea molecules have been confirmed, which are Ln³⁺⋯OC–N and Ln³⁺⋯NH₂−C. Compared with Ln³⁺⋯NH₂−C, Ln³⁺ prefers to coordinate with the OC bond in urea. With a higher concentration of rare earth ions in the solution, some N–CO⋯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.