Assessing the Role of Water on the Electronic Structure and Vibrational Spectra of Monohydrated l‑Aspartic Acid 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.