Polarization Dependence of Low-Frequency Vibrations from Multiple Faces in an Organic Single Crystal

Optical Polarization-Based Measurement Methods for Characterization of Self-Assembled Peptides' and Amino Acids' Micro- and Nanostructures

In recent years, self-assembled peptides’ and amino acids’ (SAPA) micro- and nanostructures have gained much research interest. Here, description of how SAPA architectures can be characterized using polarization-based optical measurement methods is provided. The measurement methods discussed include: polarized Raman spectroscopy, polarized imaging microscopy, birefringence imaging, and fluorescence polarization. An example of linear polarized waveguiding in an amino acid Histidine microstructure is discussed. The implementation of a polarization-based measurement method for monitoring peptide self-assembly processes and for deriving molecular orientation of peptides is also described.

Identification of Enantiomers Using Low-Frequency Raman Spectroscopy

Distinguishing between d and l enantiomers is of important scientific interest, especially for the pharmaceutical industry. Enantiomeric differentiation in the solid form is repeatedly presented as a challenge in the research community. Raman spectroscopy is a nondestructive tool, widely used for the characterization of different materials by probing their vibrational modes. The low-frequency region of the Raman spectrum reveals lattice-level interactions and global fluctuations in the molecule. Lower frequencies correspond to vibrations arising from weaker bonds and long-range interactions and hence are very susceptible to polarization changes. This work presents low-frequency Raman (LFR) spectroscopy as a facile technique to identify enantiomers. The optical setup of conventional Raman spectroscopy is engineered such that the excitation and collection geometries use an asymmetrical focal cone. In addition, a half-wave retarder is added to the excitation path and a Glan-Taylor polarizer is added to the collection path, and these modifications allow us to select the polarization plane for both excitation and collection geometries. The asymmetry in the foci when using a polarized beam for excitation provides different intensities of the collected signal for each polarization plane. In a calibrated system, one can define the chirality of an analyte by comparing the intensity of the LFR signal along orthogonal sets of polarization planes. For nonchiral molecules, the spectral intensity is always higher in the co-polarized plane when compared to the orthogonally depolarized plane, as expected. This contrast in the intensity of Raman spectra serves as a distinct tool for identifying enantiomers.

Combining polarized low-frequency Raman with XRD to identify directional structural motifs in a pyrolysis precursor

Long-range structures and dynamics are central to coordination chemistry, yet are hard to identify experimentally. By combining polarized low-frequency Raman spectroscopy with single crystal XRD to study barium nitrilotriacetate, a metal-organic coordination polymer and a useful pyrolysis precursor, we could assign Raman peaks experimentally to layer shear motions and perpendicular hydrogen bond vibrations. These directional long-range interactions further determined the preferred fracture directions during crystallization, establishing an important link between structural motifs in the precursor, and the porosity of the carbon it yields upon pyrolysis.

Raman Spectroscopy Study of Phosphorites Combined with PCA-HCA and OPLS-DA Models

Phosphorite is a nonrenewable strategic resource, a convenient and rapid method of phosphorite grade identification and classification is important to improve phosphate utilization. In this study, Raman spectroscopy has been combined with principal component analysis and hierarchical clustering analysis (PCA-HCA) and orthogonal partial least squares-discriminant analysis (OPLS-DA) models for an investigation of different grade phosphorite samples. Both the PCA-HCA and OPLS-DA models showed that different grade phosphorite samples can be clearly distinguished by statistically analyzing the Raman spectra after smoothing, baseline correction, and first derivation. In addition, the S-line study on the OPLS-DA model clearly demonstrated that the symmetrical stretching vibrational mode of phosphate near 960 cm−1 had a much more significant contribution than other vibrational modes for the differentiation of different grade phosphorite samples.