Raman scattering ofL-valine crystals

Abstract

Single crystals of L-valine, C 5 H 11 O 2 N, have been studied by Raman spectroscopy at temperatures from 17 to 300 K over the spectral range 20-3300 cm −1 . A tentative assignment of the bands is given. Careful analysis of the Raman spectra suggests that a series of changes in the low-wavenumber region may be associated with a phase transition undergone by the material, and this possibility is discussed.

High temperature Raman spectra of L-leucine crystals

“…In the L-leucine spectrum in the z(xx)z scattering geometry it is observed two bands at 245 and 288 cm −1 that can be associated with out-of-plane vibration of CH, γ (CH), and CH 3 torsion, τ(CH 3 ), respectively [14]. The low intense band at 332 cm −1 is tentatively assigned as a NCC deformation, δ(NCC) [11], while the bands at 352, 406, 445 and 460 cm −1 are assigned as skeletal structure deformations, δ(skel.) [14].…”

“…The conformations of the two leucine molecules are similar but not identical to each other and to that in DL-leucine. According to Harding and Howieson [13], the carboxyl and amino groups are hydrogen bonded in a double layer very like that in other non-polar Lamino acids (for example, L-valine [11] and L-isoleucine [8]). In terms of the irreducible representations (IRep) of the C 2 factor group, the normal modes are decomposed as Γ = 132 A + 132 B, and because there are two acoustic modes at the B IRep and one at the A IRep, the optical modes are Γ op = 131 A + 130 B.…”

“…The two bands at 838 and 849 cm −1 present an inversion of intensity for the two scattering geometries; they are assigned as out-ofplane vibration of CO − 2 , γ (CO − 2 ), and rocking of CH 3 , r(CH 3 ) [8]. The 900 -1100 cm −1 spectral region is characterized by bands associated to several CC and CN stretching vibrations, ν(CC) and ν(CN) [8,11]. The band observed at 1131 cm −1 is assigned as rocking of NH + 3 unit, r(NH + 3 ), as well as the doublet at 1177 and 1187 cm −1 (in the z(xx)z scattering geometry) [11].…”

“…They were obtained as colorless tiny platelets, similarly with L-valine [11] and L-isoleucine [8] crystals. As occurs for L-isoleucine crystal, the large face (001) of the crystal is perpendicular to the c-axis and this was defined as the z-axis through X-ray diffraction.…”

“…For the other two aliphatic amino acid crystals the picture is as follows. A temperature investigation on L-valine crystal using Raman spectroscopy technique showed that the material undergoes a phase transition between 100 and 120 K [11]. For L-leucine crystal, to the best of our knowledge, only Boygeard have published a work reporting properties of L-leucine crystal at high temperatures [12].…”