What Rules the Relative Stability of α-, β-, and γ-Glycine Polymorphs?

Theoretical calculations based on the density functional theory, using the PBE functional with the D3 dispersion correction under periodic boundary conditions, have been employed aiming to investigate the properties of alpha-, beta-, and gamma-glycine. Structural parameters have been predicted with a maximum error of 1.42% for lattice parameters and 2.53% for the unit-cell volume, for the alpha phase. Band structure calculations suggest the band gap values of 4.80, 5.01, and 5.23 eV for the alpha-, beta-, and gamma phases, respectively. Quasi-harmonic calculations have been performed and the Gibbs free energy function has been calculated in a wide range of temperature and pressures, suggesting the stability ordering gamma > alpha > beta, at room temperature, and the gamma to alpha-glycine phase transition temperature of 442.55 K, at 1 bar, in agreement with the experimental findings. Moreover, a deviation from the experimental value of only 0.44 J mol(-1) K-1 is observed for the predicted S-(alpha ->gamma) at 298.15 K. Finally, calculated sublimation enthalpies of 140.58, 138.09, and 141.70 kJ mol(-1) (alpha-, beta-, and gamma-glycine, respectively), at 298.15 K and 1 bar, have also shown good agreement with the experimental values.