Conformation and Hydrogen-Bond-Assisted Polymerization in Glycine Lithium Sulfate at High Pressures

The conformation of glycine has been a subject of extensive research for the past several years. As glycine exists in zwitterionic form in liquids and solids, the experimental observations of its neutral conformation are very limited. The complexes of glycine are simple prototypes to study the conformational properties of glycine. We have investigated the high-pressure behavior of glycine lithium sulfate (GLS), a semiorganic complex of glycine using X-ray diffraction, Raman spectroscopy, and density functional theory (DFT)-based first principles calculations. Our Raman studies and DFT calculations suggest formation of an intramolecular hydrogen bond at higher pressures. Subsequent to a structural transformation to a new high-pressure phase at similar to 9 GPa, the observed spectral changes in the Raman spectra above 14 GPa indicate possible conformational change of glycine from zwitterionic to neutral form. At pressures above 18 GPa, the characteristic features in the Raman spectra and the X-ray diffraction patterns suggest transformation to a hydrogen-bond-assisted polymeric phase with intermediate range order.