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

Abstract

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 α-, β-, and γ-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 α phase. Band structure calculations suggest the band gap values of 4.80, 5.01, and 5.23 eV for the α, β, and γ 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 γ > α > β, at room temperature, and the γ to α-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(α→γ) at 298.15 K. Finally, calculated sublimation enthalpies of 140.58, 138.09, and 141.70 kJ mol–1 (α, β, and γ-glycine, respectively), at 298.15 K and 1 bar, have also shown good agreement with the experimental values.

Glycine: The Gift that Keeps on Giving

“…The conclusions based on calorimetry data reported by different groups [66][67][68][69][70][71][72] seem to agree with each other, and are consistent with model lattice energy calculations. [73] At ambient pressure the γ-polymorph is the stable phase below 440 K, even though it is not the densest polymorph. [74] The densest α-polymorph is the thermodynamically most stable form only at temperatures above 440 K. Finally, the least dense β-polymorph is metastable across the whole temperature range.…”

“…[138] At the same time, distortions, rearrangements, or breaking of the hydrogen bonds obviously correlate with the anisotropy of strain on cooling or on hydrostatic compression, and can determine the structural mechanism of a polymorphic transition (Figures 7, 8). [73,[139][140][141] In particular, the characteristics of the hydrogen bonds determine if a phase transition will preserve the single crystal intact, if the transformation can be reversible, and if the transformation will depend noticeably on the compression rate or generally on the compression -decompression protocol. If van der Waals interactions are the "universal glue" that holds structures together, the hydrogen bonds are the "universal springs and junctions" that account for the reversible structural compression and rearrangements.…”

“…[52,[127][128][129][130][131][132] The number of theoretical works dedicated to the interactions of glycine with its environment in the gas phase, various fluids and in various crystal structures is as large as that of various experimental studies on this compound. [48,50,52,73,91,93,[133][134][135] However, it is hardly possible to select an obviously dominating type of the interaction. In a text-book by Kitaigorodsky [136] a layer in the glycine structure is shown as an example of a "close packing principle", so that van der Waals interactions are optimized.…”

“…The main assumption underlying all the proposed methods is that the relative thermodynamic stability of the glycine polymorphs is well-known as β < α < γ below 440 K and β < γ < α above 440 K, [66][67][68][69][70][71]73] and therefore any metastable polymorph must be obtained based on kinetic control of the relative rates of nucleation and subsequent crystal growth. The methods of this kinetic control can be different.…”

Understanding the Salt-Dependent Outcome of Glycine Polymorphic Nucleation

“…In an aqueous solution, glycine molecules exist as zwitterions ( + NH 3 CH 2 COO − ). In solid state under ambient conditions, it has three polymorphs [ 15 ]: the most stable γ-glycine (γ-form), the metastable α-glycine (α-form) and the least stable β-glycine (β-form), belonging to space group P3 1 (or P3 2 ), P2 1 /n and P2 1 , respectively (refer to Table S1 in the Supplementary Materials for more properties [ 16 , 17 , 18 ] of these three polymorphs). Glycine solids have applications in treating health problems [ 19 ].…”