Abstract
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 Schaller 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). Delta-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.
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