Elucidation of Chiral Symmetry Breaking in a Racemic Polymer System with Terahertz Vibrational Spectroscopy and Crystal Orbital Density Functional Theory

Low-Frequency Vibrational Spectra of 4-Fluorophenol Studied by THz Spectroscopy and Solid-State Density Functional Theory

Hydrogen bonds serve as important intermolecular interactions organizing the spatial arrangement of molecular crystals. Determining the hydrogen-bond orientations remains a challenging task. In the previous XRD study, the authors assumed a single crystal structure of 4-fluorophenol in which molecules form hexamer-ring clusters via anticlockwise hydrogen bonding. However, the existence of the other structure, which adopts clockwise hydrogen bonding, remains uncertain and warrants further exploration. We unveil distinctive fingerprint information associated with both structures by using high-resolution terahertz spectroscopy. The distinct structures result in different intramolecular geometries of 4-fluorophenol regarding the O-H bond configurations, which are manifested by a noticeable peak splitting in the 20-200 cm-1 frequency range. This result illustrates the sensitivity of THz spectroscopy to hydrogen-bond conformational polymorphism. Our findings represent a significant advancement in utilizing high-resolution THz spectroscopy to resolve hydrogen-bond orientations in molecular crystals with possible broad applicability across diverse hydrogen-bonded organic and inorganic framework materials.

Where do the Fluorine Atoms Go in Inorganic-Oxide Fluorinations? A Fluorooxoborate Illustration under Terahertz Light

The substitution of fluorine atoms for oxygen atoms/hydroxyl groups has emerged as a promising strategy to enhance the physical and chemical properties of oxides/hydroxides in fluorine chemistry. However, distinguishing fluorine from oxygen/hydroxyl in the reaction products poses a significant challenge in existing characterization methods. In this study, we illustrate that terahertz (THz) spectroscopy provides a powerful tool for addressing this challenge. To this end, we investigated two fluorination reactions of boric acid, utilizing MHF2 (M=Na, C(NH2)3) as fluorine reagents. Through an interplay between THz spectroscopy and solid-state density functional theory, we have conclusively demonstrated that fluorine atoms exclusively bind with the sp3-boron but not with the sp2-boron in the reaction products of Na[B(OH)3][B3O3F2(OH)2] (NaBOFH) and [C(NH2)3]2B3O3F4OH (GBF2). Based on this evidence, we have proposed a reaction pathway for the fluorinations under investigation, a process previously hindered due to structural ambiguity. This work represents a step forward in gaining a deeper understanding of the precise structures and reaction mechanisms involved in the fluorination of oxides/hydroxides, illuminated by the insights provided by THz spectroscopy.

The fingerprints of nifedipine/isonicotinamide cocrystal polymorph studied by terahertz time-domain spectroscopy

Cocrystal is constructed to improve physicochemical properties of active pharmaceutical ingredient and prevent polymorphism via intermolecular interactions. However, recent examples on cocrystal polymorphs display significantly different properties. Even though some analytical techniques have been used to characterize the cocrystal polymorphic system, it remains unclear how intermolecular interactions drive and stabilize the structure. In this work, we study the cocrystal polymorphs of nifedipine (NFD) and isonicotinamide (INA) using terahertz (THz) spectroscopy. Form I and form II of NFD-INA cocrystals show spectral fingerprints in THz region. Temperature-dependent THz spectra display distinguished frequency shifts of each fingerprint. Combined with solid-state density functional theory (DFT) calculations, the experimental fingerprints and their distinct responses to temperature are elucidated by specific collective vibrational modes. The vibrations of hydrogen bonding between dihydropyridine ring of NFD and INA are generally distributed below 1.5 THz, which play important roles in stabilizing cocrystal and preventing the oxidation of NFD. The rotations of methyl group in NFD are widely distributed in the range of 1.5-4.0 THz, which helps the steric recognition. The results demonstrate that THz spectroscopy is a sensitive tool to discriminate cocrystal polymorphs. It has the potential to be used as a non-invasive technique for pharmaceutical screening.

Novel Terahertz Spectroscopy Technology for Crystallinity and Crystal Structure Analysis of Cellulose

Crystallinity is an essential indicator for evaluating the quality of fiber materials. Terahertz spectroscopy technology has excellent penetrability, no harmful substances, and commendable detection capability of absorption characteristics. The terahertz spectroscopy technology has great application potential in the field of fiber material research, especially for the characterization of the crystallinity of cellulose. In this work, the absorption peak of wood cellulose, microcrystalline cellulose, wood nano cellulose, and cotton nano cellulose were probed in the terahertz band to calculate the crystallinity, and the result compared with XRD and FT-IR analysis. The vibration model of cellulose molecular motion was obtained by density functional theory. The results showed that the average length of wood cellulose (WC) single fiber was 300 μm. The microcrystalline cellulose (MCC) was bar-like, and the average length was 20 μm. The cotton cellulose nanofiber (C-CNF) was a single fibrous substance with a length of 50 μm, while the wood cellulose nanofiber (W-CNF) was with a length of 250 μm. The crystallinity of cellulose samples in THz was calculated as follows: 73% for WC, 78% for MCC, 85% for W-CNF, and 90% for C-CNF. The crystallinity values were obtained by the three methods which were different to some extent. The absorption peak of the terahertz spectra was most obvious when the samples thickness was 1 mm and mixed mass ratio of the polyethylene and cellulose was 1:1. The degree of crystallinity was proportional to the terahertz absorption coefficients of cellulose, the five-movement models of cellulose molecules corresponded to the five absorption peak positions of cellulose.

Synthesis and stereocomplex formation of enantiomeric alternating copolymers with two types of chiral centers, poly(lactic acid-alt-2-hydroxybutanoic acid)s

Stereocomplex (SC) formation was reported for the first time for enantiomeric alternating copolymers consisting of repeating units with two types of chiral centers, poly(lactic acid-alt-2-hydroxybutanoic acid)s [P(LA-alt-2HB)s]. l,l-Configured poly(l-lactic acid-alt-l-2-hydroxybutanoic acid) [P(LLA-alt-l-2HB)] and d,d-configured poly(d-lactic acid-alt-d-2-hydroxybutanoic acid) [P(DLA-alt-d-2HB)] were amorphous. Blends of P(LLA-alt-l-2HB) and P(DLA-alt-d-2HB) were crystallizable and showed typical SC-type wide-angle X-ray diffraction profiles similar to those reported for stereocomplexed blends of poly(l-lactic acid) and poly(d-lactic acid) homopolymers and of poly(l-2-hydroxybutanoic acid) and poly(d-2-hydroxybutanoic acid) homopolymers, and of l,l-configured poly(l-lactic acid-co-l-2-hydroxybutanoic acid) [P(LLA-co-l-2HB)] and d,d-configured poly(d-lactic acid-co-d-2-hydroxybutanoic acid) [P(DLA-co-d-2HB)] random copolymers. The melting temperature values and melting enthalpy values at 100% crystallinity for stereocomplexed solvent-evaporated and precipitated P(LLA-alt-l-2HB)/P(DLA-alt-d-2HB) blends were correspondingly 187.5 and 187.9 °C, and 98.1 and 91.8 J g⁻¹. Enantiomeric polymer blending of P(LLA-alt-l-2HB) and P(DLA-alt-d-2HB) can confer crystallizability by stereocomplexation and the biodegradable materials with a wide variety of physical properties and biodegradability are highly expected to be prepared by synthesis of alternating copolymers of various combinations of two types of chiral α-substituted 2-hydroxyalkanoic acid monomers and their SC crystallization.

Stereocomplex formation was reported for alternating copolymers of chiral α-substituted 2-hydroxyalkanoic acids which can be utilized for preparation of biodegradable materials with a variety of physical properties and biodegradability.

Terahertz Fingerprints of Short-Range Correlations of Disordered Atoms in Diflunisal

This work proposes a terahertz (THz) spectroscopy approach to the investigation of one of the outstanding problems in crystallography-the structure analysis of a crystal with disorder. Form I of diflunisal, in which the two ortho sites on one phenyl ring of diflunisal show occupational disorder, was used for an illustration. THz radiation interacts with the collective vibrations of correlated disorder, thus providing a promising tool to examine the symmetry of short-range correlations of disordered atoms. Through a thorough examination of the selection rule of THz vibrations in which the disordered atoms are involved to different extents, we deduced that only four short-range correlation possibilities of disorder exist and all of them display unambiguous fingerprint peaks in the 50-170 cm^-1 frequency region. We finally proposed an alternating packing model in which the correlation lengths of disorder are on the nanometer scale.

Synthesis, stereocomplex crystallization and homo-crystallization of enantiomeric poly(lactic acid-co-alanine)s with ester and amide linkages

α-Hydroxy acid and α-amino acid-derived poly(l-lactic acid-co-l-alanine)s and poly(d-lactic acid-co-d-alanine)s with different alanine unit contents were synthesized and their stereocomplex crystallization and homo-crystallization were investigated.

Synthesis, properties, and crystallization of the alternating stereocopolymer poly(l-lactic acid-alt-d-lactic acid) [syndiotactic poly(lactic acid)] and its blend with isotactic poly(lactic acid)

The alternating stereocopolymer poly(l-lactic acid-alt-d-lactic acid) or syndiotactic poly(lactic acid) was successfully synthesized by the condensation of the l-lactic acid-d-lactic acid dimer.