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 alpha-, beta-, and gamma-glycine with P2(1/n), P2(1), P3(1) symmetries, respectively. In addition, we report three new orthorhombic (o), tetragonal (t), and monoclinic (m) crystals with P2(1)2(1)2(1), P4(3), and P2(1) 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 center dot center dot center dot O, OH center dot center dot center dot O as well as secondary CH center dot center dot center dot 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.