Comparative Study on Mid-IR and Raman Spectra of Chiral Molecular Enantiomer D- and L-Xyloses

The research results show that in the mid-IR region, it is indeed difficult to identify the chirality of this pair of enantiomers from the perspective of band frequency, relative intensity, and profile, which is consistent with most studies. However, in the far-IR region, corresponding to electromagnetic wave spectrum THz region of Raman shift, Raman scattering spectroscopy exhibits a certain degree of characteristic differences, indicating that precise traditional low-frequency Raman scattering experiments and expected far-IR/THz absorption spectra can be used as tools for the chiral recognition of this pair of molecular enantiomers. Figure 1. (a) Haworth configuration formula and (b) common 4C1 chair conformation formula of -D/L- lyxopyranose enantiomers; Figure 2. The whole maximal-value-normalized spectral profiles of (a) mid-IR (4000–400 cm–1) and (b) Raman spectra (4000–50 cm–1) of D-xylose and L-xylose enantiomers, respectively; Figure 3. The amplified high-frequency section (3500–2800 cm–1) of (a) mid-IR and (b) Raman spectra of D-xylose and L-xylose, respectively. The black values belong to D-xylose, red ones to L-xylose, and blue ones commonly to both enantiomers; Figure 4. The amplified middle-frequency section (1500–700 cm–1) of (a) mid-IR and (b) Raman spectra of D-xylose and L-xylose, respectively. The black values belong to D-xylose, red ones to L-xylose, and blue ones commonly to both enantiomers; Figure 5. The amplified low-frequency section (700–50 cm–1) of (a) mid-IR and (b) Raman spectra of D-xylose and L-xylose, respectively. The black values belong to D-xylose, red ones to L-xylose, and blue ones commonly to both enantiomers;Table 1. Spectral band data (wavenumber: cm‒1) of solid D/L-xylose powders. The values in the brackets at L-xylose columns are the differences between the wavenumber values of the L-xylose and D-xylose. Letters “s”, “m” and “w” following the values present relatively “strong”, “moderate” and “weak” intensities of corresponding bands, respectively. Specially, the strong band values are in bold writing. The numbers in the column “Band No.” correspond to all the 54 theoretical band numbers of L-xylopyanose given in Ref.[67]. All the observed bands that may correspond to those of Ref.[67] can be assigned according to the column “Vibrational assignments” in Table 2 of Ref.[67].