In Situ Ultralow-Frequency Raman Tracking of the Polymorphic Transformation of Crystalline 1,1′-Binaphthyl

Abstract

Extremely low-frequency (LF) Raman spectroscopy that allows for fast access to lattice vibrations with little hindrance of immense Rayleigh scattering was used to track in situ thermally induced polymorphic transformation of crystalline 1,1′-binaphthyl (BN), a fundamental structural unit of specific chiral ligands. BN occurs in two conformational polymorphs, cisoid and transoid forms, with the former being a racemate and the latter being a conglomerate of chiral crystals. Observed LF Raman spectra (from 200 cm–1 down to ∼5 cm–1, equivalent to 0.15 THz), which arise predominantly from lattice vibrations, show markedly different features between the cisoid and transoid forms, making it possible to study their phase transition. A series of LF Raman spectra measured with slow heating of microcrystalline BN reveal that, in contrast to previous studies, the monoclinic cisoid form transforms directly to the tetragonal transoid form in the solid state without formation of any intermediate phase. The present approach is applicable to various phase transition systems, including pharmaceutical compounds, liquid crystals, and polymer films, which conventional X-ray diffraction analysis may not be able to investigate.

Complementing Crystallography with Ultralow‐Frequency Raman Spectroscopy: Structural Insights into Nitrile‐Functionalized Ionic Liquids

“…Figure1compares the LFR spectra of the five cbmim salts. [19,20,33,34] Finally,t he LFR spectral pattern of cbmimNTf 2 (Figure 1c)r esembles neither those of cbmimBra nd cbmimN(CN) 2 nor those of cbmimBF 4 and cbmimPF 6 .C orrespondingly,w hereas cbmimBr,c bmimBF 4 ,a nd cbmimPF 6 belongt ot he monoclinic space group P2 1 /c or P2 1 /n,o nly cbmimNTf 2 hast he lower symmetry of the triclinic space group P " TheL FR spectral patterns enabled us to classify the five cbmim salts into three groups.c bmimBr ( Figure 1a)a nd cbmimN(CN) 2 (Figure 1b)b oth have two sharpb ands below 40 cm À1 and several broad bands between 40 and 150 cm À1 , exhibiting ar emarkable overall similarity.L ikewise, apart from the differences in exact peak positions, the LFR spectrao f cbmimBF 4 (Figure 1d)a nd cbmimPF 6 ( Figure 1e)a lso look very much alike.…”

“…The same conclusion may be reached by looking at higher-frequency (> 200 cm À1 )R aman spectra,b ut intramolecular vibrations therein reflectt he crystal structure only indirectly.F urthermore, in crystals, the Raman intensities of lattice vibrations are typically much stronger (by af actor of 5-10 or more) than those of intramolecular vibrations, [20] thus facilitating fast detection. It Scheme1.Chemical structure of cbmimX, where X = Br,N(CN) 2 ,NTf 2 ,BF 4 , and PF 6 .…”

“…[19,20] As low-frequency Raman( LFR) spectra of crystalline solids arise mainly from collectivea nd intermolecular motions, they yield in-depth insights into intermolecular interactions (e.g.,C oulombi nteraction, hydrogen bonding, and p-p interactions)a nd the resulting crystal packing that determine the physicalp roperties of the crystals. In previous studies, we have developed af ast low-frequency multichannel Ramans pectrometer that can access Raman spectra close to the Rayleigh line (down to approximately 5cm À1 )and have applied the developedmethod to investigate conformational polymorphs and polycrystalline microstructures of molecular crystals of 1,1'-binaphthyl.…”

“…In previous studies, we have developed af ast low-frequency multichannel Ramans pectrometer that can access Raman spectra close to the Rayleigh line (down to approximately 5cm À1 )and have applied the developedmethod to investigate conformational polymorphs and polycrystalline microstructures of molecular crystals of 1,1'-binaphthyl. [19,20] As low-frequency Raman( LFR) spectra of crystalline solids arise mainly from collectivea nd intermolecular motions, they yield in-depth insights into intermolecular interactions (e.g.,C oulombi nteraction, hydrogen bonding, and p-p interactions)a nd the resulting crystal packing that determine the physicalp roperties of the crystals. [21] We now extend fast ultralow-frequency Raman spectroscopyt oionic crystals and liquidso fp racticali nterest.…”

In-line ATR-UV and Raman Spectroscopy for Monitoring API Dissolution Process During Liquid-Filled Soft-Gelatin Capsule Manufacturing

“…Raman spectroscopy has been found in a wide array of applications in the pharmaceutical industry, including inprocess monitoring of various processes such as synthetic organic reactions, API crystallization, granulation and coating (12,24,29,30). The high specificity of Raman spectral features makes it a suitable tool for analyzing multi-component systems both qualitatively and quantitatively.…”