In situ Raman study of the crystallization of glycine

Abstract

The process of crystallization of organic molecules is a well-known effect. However, a full understanding of this process is still lacking because it is very challenging to monitor fast changes in the molecular interactions. In this work, we perform an in situ study on the crystallization of glycine upon H₂O and D₂O evaporation by micro-Raman spectroscopy. The magnitude of the frequency splitting between the high-frequency asymmetric and the low-frequency symmetric CH₂ stretching components is used to determine the dihedral angle ξ, which measures the deviation from the planar glycine conformation through rotation around the C–C bond, hence providing insights on the molecular arrangement. Our results show that the average ξ of the glycine molecules in both H₂O and D₂O is ∼20°, and it does decrease to ∼18° when the mature crystal is formed, corresponding to the molecular arrangement of α-glycine. However, right before the crystal is fully formed, a complex CH peak lineshape is observed, indicating that some of the molecules have ξ ∼22°, which is close to the molecular arrangement of β-glycine. Our results show that glycine molecules in water do not crystallize directly into a perfect α-glycine crystal, but a metastable state is first formed, where some of the molecules are more bent as compared to the α-glycine configuration.