Abstract
The infrared absorption and Raman scattering spectra of the monoclinic
P2(1) L-aspartic acid anhydrous crystal were recorded and interpreted
with the help of density functional theory (DFT) calculations. The
effect of dispersive forces was taken into account, and the optimized
unit cells allowed us to obtain the vibrational normal modes. The
computed data exhibits good agreement with the measurements for low
wavenumbers, allowing for a very good assignment of the infrared and
Raman spectral features. The vibrational spectra of the two lowest
energy conformers of the L-aspartic molecule were also evaluated using
the hybrid B3LYP functional for the sake of comparison, showing that the
molecular calculations give a limited description of the measured IR and
Raman spectra of the L-aspartic acid crystal for wavenumbers below 1000
cm(-1). The results obtained reinforce the need to use solid-state
calculations to describe the vibrational properties of molecular
crystals instead of calculations for a single isolated molecule picture
even for wavenumbers beyond the range usually associated with lattice
modes (200 cm(-1) < omega < 1000 cm(-1)).
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