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

Nemtsov Nemtsov,Mastai Mastai,Aviv Aviv,Tischler Tischler

doi:10.3390/cryst9080425

Abstract

Recent developments in optical filters have enabled the facile use of Raman spectroscopy for detection of low frequency (LF) vibrational modes. LF-Raman spectroscopy offers fast and sensitive characterization of LF vibrations, and enables the measurement of single microcrystals and detection of defects. It is useful for probing intermolecular interactions in crystals, which are lower in energy, such as hydrogen bonds, shear modes, and breathing modes. Crystal excitation from multiple faces allows learning the orientation of intermolecular interactions, as polarization dependence varies with the polarizability of the interactions along the planes. Elucidating the orientations of the intermolecular interactions in organic crystals is essential for guiding the reactions or adsorption to a specific crystal face. In this study, we investigated the dependence of the LF-Raman signal intensity on the orientation of an organic single microcrystal of L-alanine. Three incident beam directions provided the orientations of the intermolecular interactions by analyzing the corresponding LF-Raman spectra. The signal intensity correlated well with the proximity between the incident beam’s direction and the orientations of the intermolecular interactions. Excellent compatibility was found between the spectra and simulated orientations based on structural information.

Highlights

Under proper conditions, pure compounds ranging from small molecules to organometallic complexes, proteins, and polymers are organized into highly ordered crystal structures
We investigated the crystallographic orientation of an L-alanine single microcrystal as a model organic crystal using LF-Raman spectroscopy
We showed that with LF-Raman, it is possible to detect intermolecular interactions in the crystal-hydrogen bonds and shear modes

Summary

Introduction

Pure compounds ranging from small molecules to organometallic complexes, proteins, and polymers are organized into highly ordered crystal structures. During crystallization, these compounds typically adopt a unique three-dimensional orientation. The crystal structure affects many physical and chemical properties. The structure may have an impact on pharmaceutical properties [1,2,3]. It is extremely important to investigate and understand the structure and orientations of intermolecular interactions in crystals. Many different physical techniques have been used to characterize crystalline structures, such as X-ray diffraction (XRD) [4], thermal analysis [5], and electron microscopy [6]. Low-frequency (LF) Raman was presented as a tool for chiral characterization of crystalline powders [7,8]

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