No abstract

Density functional calculations were performed to study the properties of the three main glycine solid-state polymorphs α, β, and γ and the glycine dihydrate (GDH) crystal. Optimized unit cell geometries, Kohn–Sham electron energy bands, electron densities of states, population charges, carrier effective masses, optical absorption, and complex dielectric functions were obtained for each glycine system using a GGA functional plus the TS dispersion correction, leading to lattice parameters very close to the experimental values. The theoretical fundamental gaps of all glycine crystals are indirect and near 5 eV. The carrier effective masses are anisotropic and especially heavy (∼26 free electron masses at most) for holes in the case of the GDH crystal. The optical absorption and the dielectric function are very sensitive to the polarization of light in all glycine-based crystals.

In this work, we compare the calculated electronic and vibrational properties (infrared and Raman spectra) of anhydrous and monohydrated L-aspartic acid crystals to unveil the role of water in the later. This is accomplished through density functional theory (DFT) simulations within the Tkatchenko and Scheffler dispersion corrected generalized gradient approximation (GGA+TS). Both crystals are shown to have indirect band gaps and the simulations predict that water has a small effect on the Kohn− Sham band gap value (60 meV). Δ-sol corrected gaps, in contrast, exhibited a larger difference between the anhydrous and monohydrated crystals (main gap 0.30 eV larger for the latter). The conduction bands of the monohydrated species are much flatter because of structural changes produced by the presence of water, which leads to very large electron effective masses. However, the hole effective mass along the stacking direction of water molecules in the GGA+TS optimized monohydrated crystal is smaller than in other directions, suggesting that water stacking can improve on hole transport in similar bioorganic systems. These effects highlight the complex role of water on the carrier transport properties in monohydrated L-aspartic acid crystals, which is in contrast with the general belief that water simply increases the electrical conductance in molecular crystals. Finally, the calculated infrared and Raman spectra of the monohydrated phase exhibit molecular water vibrational signatures as well as water related peak shifts of as much as 100 cm −1 in comparison to those of the anhydrous structure. Remarkable water related Raman intensity peaks were obtained for the monohydrated crystal in the wavenumber ranges between 600 and 1000 cm −1 and between 2350 and 3450 cm −1 , while for the infrared spectrum, a set of water absorption bands can be clearly identified in the 1550−1750 and 2800−3400 cm −1 wavenumber intervals.