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

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, KH₂PO₄, 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 KH₂PO₄ from aqueous solution becomes clear. In KH₂PO₄ aqueous solution, there are hydrated status, (H₂PO₄^–)_<i>n</i> 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 KH₂PO₄ have been distinguished. At the initial stage of aggregation formation, P–O bond in H₂PO₄^– group guides the P–O···H–O–P hydrogen bonding, leading to the H₂PO₄^– that retains <i>C</i>_2<i>v</i> symmetry, whereas P–O···H–O–P hydrogen bonding guides the twist and rotation of H₂PO₄^– groups in prenucleation clusters, promoting the local structural evolution of (H₂PO₄^–)_<i>n</i> from <i>C</i>_2<i>v</i> to <i>D</i>_2<i>d</i> and the formation of crystalline KH₂PO₄ nuclei. The present work deepens the hydrogen bonding effect that can warrant much space to adjust the chemical bonding environment in constructing crystallographic frames