Hydrogen Bonding Paradigm in the Formation of Crystalline KH₂PO₄ from Aqueous Solution

Abstract

Revealing the hydrogen bonding paradigm is critical to clarify the formation mechanism of hydrogen bonded materials. The nucleation process of a typical nonlinear optical crystal, KH 2 PO 4 , is identified by in situ molecular vibration spectroscopy, which effectively demonstrates the oriented role of hydrogen bonding in local structure engineering. On the basis of the vibrational evolution of hydrogen bonds, the partition of different periods in the formation of crystalline KH 2 PO 4 from aqueous solution becomes clear. In KH 2 PO 4 aqueous solution, there are hydrated status, (H 2 PO 4 − ) n aggregations, and prenucleation clusters. The prenucleation clusters exist in the solution with a metastable status over a period of time, and then they will transform into crystalline ones within a short time. Two distinct roles of P−O•••H−O−P hydrogen bonding in the formation of crystalline KH 2 PO 4 have been distinguished. At the initial stage of aggregation formation, P−O bond in H 2 PO 4 − group guides the P−O•••H−O−P hydrogen bonding, leading to the H 2 PO 4 − that retains C 2v symmetry, whereas P−O•••H−O−P hydrogen bonding guides the twist and rotation of H 2 PO 4 − groups in prenucleation clusters, promoting the local structural evolution of (H 2 PO 4 − ) n from C 2v to D 2d and the formation of crystalline KH 2 PO 4 nuclei. The present work deepens the hydrogen bonding effect that can warrant much space to adjust the chemical bonding environment in constructing crystallographic frames.

Influence of molecular symmetry on multiple pathways of crystallization

“…Using this method, the structural evolution of both KDP and ADP aqueous solutions is successfully detected as a function of supersaturation. The measurements reveal that both solutions have (H 2 PO 4 − ) 2 dimer unit block as the solute structure, even at low concentrations, which has been indirectly predicted in gas phase by calculation study 37,40 , but differs from general expectations in solution studies 41,42 . Interestingly, we nd that the solute structure abruptly changes at high supersaturation in KDP solution, which is accompanied by the molecular symmetry changes in H 2 PO 4 ions from C 2v to C 1 as supersaturation increases.…”

“…From the position and shape of G3 and G4 peaks, we can recognize the formation of large size H 2 PO 4 − clusters with diameters greater than 1 nm in the solution, before crystal nucleation occurs. Since the cluster size is average value, we can imagine the existence of larger clusters than 1 nm in the solution which may re ect the early stage of two-step nucleation (i.e., dense liquid region) 15,41 .…”

Surface Plasmon Resonance (SPR) Combined Technology: A Powerful Tool for Investigating Interface Phenomena

“…In the field of material R & D and design, Raman spectroscopy can be used to observe the changes in the microlattice, chemical bonding mode, and molecular structure of a produced material. [161][162][163][164] SPR spectroscopy can be used to analyze the film thickness on an interface and provide kinetic parameters, which can be used to control the crystallization process of the reaction and solution. In general, SPR-Raman combination technology is still in the stage of rapid development, and people are constantly exploring the surface properties of the system and sensor.…”

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