Discerning subtle high-pressure phase transitions in glyphosate

• Published in: CrystEngComm Vol. 25; no. 6; pp. 988 - 997
• Main Authors: Wilson, Cameron J. G, Wood, Peter A, Parsons, Simon
• Format: Journal Article
• Language: English
• Published: Cambridge Royal Society of Chemistry 06.02.2023
• Subjects: Compressibility; Glycine; Herbicides; High pressure; Hydrogen bonds; Phase transitions; Pressure effects; Raman spectroscopy; Single crystals
• ISSN: 1466-8033; 1466-8033
• DOI: 10.1039/d2ce01616h

The common garden herbicide glyphosate, N -(phosphonomethyl)glycine, has been studied between ambient pressure and 5.17 GPa using single crystal X-ray diffraction. Glyphosate forms a structure composed of layers parallel to the (1 0 −2) planes. Hydrogen bonds form along the stacking direction, which are very incompressible so that the effects of pressure are accommodated mostly within the layers. This study has confirmed two high pressure phase transitions previously observed by Raman spectroscopy, enabling the structural signatures of the transitions to be identified. Both transitions are very subtle and second order, involving changes to the way the structure responds to pressure rather than changes to the structure. The first transition occurs between 0.93-1.21 GPa and corresponds to the onset of greater compressibility within the layers. The second transition between 3.78-4.23 GPa is an intramolecular feature signalling a deformation of the molecular backbone. In the absence of a first order phase transition, the packing remains in a compressed form of its ambient pressure form up until the highest pressure measured. A reconstructive phase transition occurs at 5.98 GPa forming a polycrystalline high-pressure phase. The partitioning of volumes into 'occupied' and 'unoccupied' space can recover a spectroscopic level of sensitivity in crystallographic measurements. This has revealed structural effects of two second order high pressure phase transitions in glyphosate.