Low-temperature vibrational spectra and molecular structure of L-alanine

Solid L-α-alanine: Spectroscopic properties and theoretical calculations

“…This is shown in the spectra represented in traces (a) and Table I. These assignments, in which groups of vibrations are sometimes clustered together, agree well in general with previous literature descriptions [12,13,15,19].…”

“…Those variations indicate that a reorganization of the crystal structure takes place at low temperatures, and it was suggested that this reorganization was probably caused by changes in the proton positions. In our experiment, some variations were observed upon cooling in the peaks at 1596 and 1524 cm -1 , attributed to the deformation mode of NH 3 + , but these variations, barely appreciable in Figure 2, were much smaller than those reported by Ivanov et al [15]. In general, the bands in the IR spectra do not narrow appreciably and no significant changes are seen when the temperature of the solid generated from the vapour phase is lowered (compare black and grey traces of Figure 2 (a)).…”

“…The larger width probably reflects a higher degree of amorphicity in the vapour deposited solid, whereas in the KBr matrix small alanine single crystals must be present. Further support to this hypothesis comes from the comparison with previous studies on the temperature evolution of the spectrum of polycrystalline alanine in KBr pellets by Ivanov et al [15], Barthes et al [13] and Rozenberg et al [12]. These authors observed significant spectral changes when the samples were cooled.…”

“…In general, the bands in the IR spectra do not narrow appreciably and no significant changes are seen when the temperature of the solid generated from the vapour phase is lowered (compare black and grey traces of Figure 2 (a)). Furthermore, an absorption band was observed at 972 cm -1 in the low temperature spectrum of the alanine pellets [12,13,15], which is assigned to an overtone of NH 3 + coupled to one or more phonons of the crystal lattice. This band is not detected in the spectra recorded from vapour deposited films, providing evidence for a lack of a proper crystal structure that would allow the propagation of phonons, and its subsequent spectral activity.…”

“…The neutral species can have several conformers, unravelled by high-sensitive molecular beam Raman spectroscopy [9] and FT microwave techniques [10,11]. In turn, the zwitterion can present slightly different configurations arising from the possible H-bonding interactions in which the NH 3 + group can be involved [12][13][14][15], and also depending on the pH and characteristics of the solution [16,17]. Sophisticated techniques have been devised and applied to study the solids as room temperature matrices [18].…”