New Stable Phases of Glycine Crystals

A Density Functional Theory (DFT) study of the electronic, energetic, dynamical, and mechanical properties of new glycine molecular crystals is presented here. Our search of the potential energy surface (PES) reproduces the previously reported structures for α-, β-, and γ-glycine with P21/n,P21,P31 symmetries, respectively. In addition, we report three new orthorhombic (o), tetragonal (t), and monoclinic (m) crystals with P212121,P43, and P21 symmetries. The crystals have wide band gaps, classifying them in the range of insulators. All three new phases have low mechanical hardness (< 3.2 GPa), characterizing them as soft crystals. Topological and local energetic properties of the electronic densities for the new crystalline phases of glycine have been calculated using the tools provided by the quantum theory of atoms in molecules (QTAIM) under periodic conditions. Typical NH⋯O, OH⋯O as well as secondary CH⋯O hydrogen bonds (HBs), act as the stabilizing factors resulting in large cohesive energies for the new phases of glycine crystals. Without exception, all types of HBs, for all new phases, perfectly fit the attractive region of a Lennard-Jones potential.