ICE Issue 9

10 Scientific Article The Israel Chemist and Chemical Engineer Issue 9 · January 2023 · Tevet 5783 by the chirality, and thus the hydrogen bond network is limited, whereas for racemic crystals, there is an extremely large availability of hydrogen bonding modes [16]. Hence the structure is stochastically defined. These considerable differences in the crystalline structure result in differences in the intermolecular interactions of racemic and enantiopure crystals. As a result, distinct vibrational modes exist in enantiopure and racemic crystals that are detectable by LFRS. Some examples of enantiomeric differentiation from their racemic mixtures are presented in Figure 3 and 4. Hence, it is observed that LFRS produces completely different spectra for racemic and enantiopure crystals. Moreover, LFRS offers faster and more sensitive chiral characterization in crystals than currently used methods, enabling facile measurements for microcrystals and detection of defects in chiral crystals. More detailed study can be found elsewhere [15]. Polarization dependence of LFRS in organic single crystals Crystal lization is a very unique phenomenon in pure compounds. Post nucleation, under a conducive environment, crystal lization results in formation of highly ordered structures and adaptation of a unique three-dimensional orientation. The crystal structure governs physical and chemical properties in materials [17]. Therefore, it is extremely important to investigate and understand the structure and orientations of intermolecular interactions in crystals. Over the years, many different physical techniques hav e been used to characterize crystalline structures, such as X-ray diffraction (XRD), thermal analysis, and electron diffraction. In general, each face of a single crystal provides detailed structural information. The most common experimental method that al lows resolution of individual atoms is single crystal X-ray diffraction (SCXRD) [18]. However, it requires a sufficiently large crystal that is at least partially transparent i.e. in general is bright looking, having clear edges and faces, and is free of inclusions. Another method for characterizing crystals that uses X-rays is near-edge X-ray absorption fine structure, a technique which determines the molecular orientation for non-transparent samples. Raman spectroscopy, with established higher sensitivity than XRD in crystal purity investigations, provides information on both covalent bonds based on intramolecular vibrations and intermolecular interactions [19]. At the same time, intermolecular interactions that result in shear modes, breathing modes, and hydrogen bond stretching modes are lower in energy and are observed in LFRS. The LFR modes are generated by weak interactions including molecular degrees of freedom and shear modes, and are observed in the lower range of LFRS, while vibrational modes that are generated by stronger intermolecular interactions such as hydrogen bonds exhibit larger LFR shifts within the LFRS spectral range. Theoretical and computational efforts have successfully assigned these larger LFR modes to hydrogen bond stretching vibrations using density functional theory (DFT) calculations. Previous studies have used polarized Raman spectroscopy for various applications in material characterization. Polarization dependence in Raman, along with transmission electron microscopy, is used to investigate the crystallographic orientation of dark crystals [20]. Polarization-dependent contrast in the interaction cross section of LFR modes was primarily probed in this study. We observe that investigation of crystal structures is indeed possible by studying vibrational modes obtained from each face of a single crystal using LFRS. Unlike other methods, Figure 5. (a) Photo of L-aniline single crystal and measured planes. (b) L-aniline crystal structure constructed using the program “Mercury” along with the measured planes (101), (002), and (011).