In Situ Cocrystallization via Spray Drying with Polymer as a Strategy to Prevent Cocrystal Dissociation

The aim of the present study was to investigate how different polymers affect the dissociation of cocrystals prepared by co-spray-drying active pharmaceutical ingredient (API), coformer, and polymer. Diclofenac acid-l-proline cocrystal (DPCC) was selected in this study as a model cocrystal due to its previously reported poor physical stability in a high-humidity environment. Polymers investigated include polyvinylpyrrolidone (PVP), poly(1-vinylpyrrolidone-co-vinyl acetate) (PVPVA), hydroxypropyl methyl cellulose, hydroxypropylmethylcellulose acetate succinate, ethyl cellulose, and Eudragit L-100. Terahertz Raman spectroscopy (THz Raman) and powder X-ray diffraction (PXRD) were used to monitor the cocrystal dissociation rate in a high-humidity environment. A Raman probe was used in situ to monitor the extent of the dissociation of DPCC and DPCC in crystalline solid dispersions (CSDs) with polymer when exposed to pH 6.8 phosphate buffer and water. The solubility of DPCC and solid dispersions of DPCC in pH 6.8 phosphate buffer and water was also measured. The dissociation of DPCC was water-mediated, and more than 60% of DPCC dissociated in 18 h at 40 °C and 95% RH. Interestingly, the physical stability of the cocrystal was effectively improved by producing CSDs with polymers. The inclusion of just 1 wt % polymer in a CSD with DPCC protected the cocrystal from dissociation over 18 h under the same conditions. Furthermore, the CSD with PVPVA was still partially stable, and the CSD with PVP was stable (undissociated) after 7 days. The superior stability of DPCC in CSDs with PVP and PVPVA was also demonstrated when systems were exposed to water or pH 6.8 phosphate buffer and resulted in higher dynamic solubility of the CSDs compared to DPCC alone. The improvement in physical stability of the cocrystal in CSDs was thought to be due to an efficient mixing between polymer and cocrystal at the molecular level provided by spray drying and in situ gelling of polymer. It is hypothesized that polymer chains could undergo gelling in situ and form a physical barrier, preventing cocrystal interaction with water, which contributes to slowing down the water-mediated dissociation.

Low-Frequency Raman Spectroscopy of Pure and Cocrystallized Mycophenolic Acid

The aqueous solubility of solid-state pharmaceuticals can often be enhanced by cocrystallization with a coformer to create a binary cocrystal with preferred physical properties. Greater understanding of the internal and external forces that dictate molecular structure and intermolecular packing arrangements enables more efficient design of new cocrystals. Low-frequency (sub-200 cm^-1) Raman spectroscopy experiments and solid-state density functional theory simulations have been utilized together to investigate the crystal lattice vibrations of mycophenolic acid, an immunosuppressive drug, in its pure form and as a cocrystal with 2,2'-dipyridylamine. The lattice vibrations primarily consist of large-amplitude translations and rotations of the crystal components, thereby providing insights into the critical intermolecular forces governing cohesion of the molecular solids. The simulations reveal that despite mycophenolic acid having a significantly unfavorable conformation in the cocrystal as compared to the pure solid, the cocrystal exhibits greater thermodynamic stability over a wide temperature range. The energetic penalty due to the conformational strain is more than compensated for by the strong intermolecular forces between the drug and 2,2'-dipyridylamine. Quantifying the balance of internal and external energy factors in cocrystal formation indicates a path forward in the development of future mycophenolic acid cocrystals.

Exploring the Solid-State Landscape of Carbamazepine during Dehydration: A Low Frequency Raman Spectroscopy Perspective

The solid-state landscape of carbamazepine during its dehydration was explored using Raman spectroscopy in the low- (-300 to -15, 15 to 300) and mid- (300 to 1800 cm^-1) frequency spectral regions. Carbamazepine dihydrate and forms I, III, and IV were also characterized using density functional theory with periodic boundary conditions and showed good agreement with experimental Raman spectra with mean average deviations less than 10 cm^-1. The dehydration of carbamazepine dihydrate was examined under different temperatures (40, 45, 50, 55, and 60 °C). Principal component analysis and multivariate curve resolution were used to explore the transformation pathways of different solid-state forms during the dehydration of carbamazepine dihydrate. The low-frequency Raman domain was able to detect the rapid growth and subsequent decline of carbamazepine form IV, which was not as effectively observed by mid-frequency Raman spectroscopy. These results showcased the potential benefits of low-frequency Raman spectroscopy for pharmaceutical process monitoring and control.

Low-Frequency Vibrational Spectroscopy and Quantum Mechanical Simulations of the Crystalline Polymorphs of the Antiviral Drug Ribavirin

Crystal polymorphism is a common phenomenon in pharmaceutical solids and a critical issue when considering the formulation of therapeutics since multiple polymorphs may form during drug manufacturing. Low-frequency vibrational spectroscopy is sensitive to polymorphic content, and in this work, terahertz time-domain spectroscopy and low-frequency Raman spectroscopy were utilized in the study of crystalline ribavirin, a widely applicable antiviral. Characteristic spectra with numerous peaks in the sub-200 cm^–1 region were obtained of the more common polymorph of ribavirin (Form II). Solid-state density functional theory (ss-DFT) simulations were then used to optimize the crystal structure of this polymorph and calculate the frequencies and spectral intensities of the lattice vibrations in the low-frequency region. The near-harmonic thermal behavior of the sample with cooling enabled excellent agreement between experiment and theory to be achieved, emphasizing the quality of the applied model, and the observed spectral peaks could be assigned to specific atomic motions in the solid. Form I and Form II polymorphs of ribavirin were both investigated with ss-DFT to understand the different aspects governing the relative stabilities of these solids. The ss-DFT simulations of the polymorph energies revealed that Form II is more stable at all temperatures due to a stronger cohesive energy than Form I; however, ribavirin in Form I has a significantly lower conformational energy. The finding of monotropism appears to conflict with the reported enantiotropism of the ribavirin polymorphs but ultimately confirms that crystal defects in the real samples greatly affect the thermodynamic relationship of the crystals.

In Situ Observation of the Structure of Crystallizing Magnesium Sulfate Heptahydrate Solutions with Terahertz Transmission Spectroscopy

Terahertz time-domain spectroscopy in a transmission geometry combined with visual analysis was used to investigate the crystallization process of MgSO₄ solution. Careful spectral analysis of both a feature at 1.6 THz and the overall magnitude of absorption allowed the extraction of information about the liquid phase before and during crystallization, aiding the investigation of solvation dynamics and the behavior of molecular species at phase boundaries. The method was reproducibly applied to a number of measurements on a series of solutions of three chosen concentrations at different temperatures. When increasing temperature at the end of the measurement, the dissolution of crystals was observed as well. The temperature-dependent absorption data of the semicrystalline systems were converted to the solvent concentrations using a recently developed method. Solutions of a series of concentrations were also investigated in the temperature range of 4-25 °C. The results were compared to the theoretical calculated values, and the consistent differences proved the existence of a hydration shell around the salt ions whose behavior is different from bulk water. Future work will focus on triggering nucleation at specific positions in order to study the very beginning of the crystallization process. MgSO₄ heptahydrate is used as a model system in this study, while the concept and the setup can be applied to other systems.

Study on Hydration and Dehydration of Ezetimibe by Terahertz Spectroscopy with Humidity-Controlled Measurements and Theoretical Analysis

Understanding the solid-state transitions of active pharmaceutical ingredients (APIs) is essential for quality control since differences in their forms affect the bioavailability of APIs. Terahertz (THz) frequency-domain spectroscopy is suitable for such an application since it can sensitively probe the lattice phonon modes originating in the crystal structures. THz absorption spectra were obtained for ezetimibe (EZT) and ezetimibe monohydrate (EZT-MH), which have similar crystalline structures and belong to the same space group. The observed absorption spectrum of EZT matched well with the solid-state density functional theory (ss-DFT)-simulated spectrum for the structures at 0 K and room temperature (modeled using constrained unit cell volumes). However for EZT-MH, the ss-DFT spectrum of the room-temperature structure showed better correlation with the experimental THz spectrum than that of the simulated spectrum of the 0 K structures, suggesting that the EZT-MH crystal has greater anharmonic character. Gibbs free-energy curves were calculated, and EZT-MH was found to be more stable than pure EZT and water in a broad temperature range. The hydrate stability may be influenced by the existence of more hydrogen bonds in EZT-MH. The hydration and dehydration of EZT in a pure API tablet and formulation tablets were monitored using a THz spectrometer with a humidity-controlled sample chamber. The effect of the excipient in the formulation tablet on hydration was successfully confirmed by showing that the solid-state transition of the API with excipients is significantly slower than that without it. Under a relative humidity of 60%, hydration of EZT in a pure EZT tablet occurred in 200 min, while the hydration of EZT in a formulation tablet was 50 times slower.

Photocatalytic turnover of CO2 under visible light by [Re(CO)3(1-(1,10) phenanthroline-5-(4-nitro-naphthalimide))Cl] in tandem with the sacrificial donor BIH

Optimized photocatalytic conversion of CO₂ requires new potent catalysts that can absorb visible light. The photocatalytic reduction of CO₂ using rhenium(i) has been demonstrated but suffers from low turnover. Herein, we describe a [Re(CO)₃(1-(1,10)phenanthroline-5-(4-nitro-naphthalimide))Cl] photocatalyst, which when combined with the sacrificial donor 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole, results in selective production of formic acid and a high turnover number of 533 and turnover frequency of 356 h⁻¹. Single-crystal X-ray diffraction and DFT studies are also discussed.

Rhenium based photocatalytic conversion CO₂ to formate in the visible region with excellent turnover.

New Insights into the Crystallographic Disorder in the Polymorphic Forms of Aspirin from Low-Frequency Vibrational Analysis

Terahertz time-domain spectroscopy (THz-TDS) is applied to two polymorphs of acetylsalicylic acid (aspirin), and the experimental spectra are compared to lattice dynamical calculations using high accuracy density functional theory. The calculations confirm that forms I and II have very close energetic and thermodynamic properties and also that they show similar spectral features in the far-infrared region, reflecting the high degree of similarity in their crystal structures. Unique vibrational modes are identified for each polymorph which allow them to be distinguished using THz-TDS measurements. The observation of spectral features attributable to both polymorphic forms in a single sample, however, provides further evidence to support the hypothesis that crystalline aspirin typically comprises intergrown domains of forms I and II. Differences observed in the baseline of the measured THz-TDS spectra indicate a greater degree of structural disorder in the samples of form II. Calculated Gibbs free-energy curves show a turning point at 75 K, inferring that form II is expected to be more stable than form I above this temperature as a result of its greater vibrational entropy. The calculations do not account for any differences in configurational entropy that may arise from expected structural defects. Further computational work on these structures, such as ab initio molecular dynamics, would be very useful to further explore this perspective. Here, aspirin is a model system to show how the additional insight from the low-frequency vibrational information complements the structural data and allows for quantitative thermodynamic information of pharmaceutical polymorphs to be extracted. The methodology is directly applicable to other polymorphic systems.

[Re(CO)3(5-PAN)Cl], a rhenium(I) naphthalimide complex for the visible light photocatalytic reduction of CO2

A rhenium(i) naphthalimide complex [Re(CO)3(5-PAN)Cl] (Re(5-PAN); 5-PAN = 1-(1,10-phenanthroline)-4-nitro-naphthalimide) was synthesized, characterized, and evaluated as a photocatalyst for CO2 reduction. Characterization included use of MALDI-ToF mass spectrometry, FT-IR, RAMAN, 1H and 13C NMR, elemental analysis, electronic absorption and emission spectroscopy, single crystal X-ray diffraction, DFT and cyclic voltammetry. Photocatalytic (406 nm) reduction of 13CO2 to formate (H13COO) in the presence of this catalyst was tracked via13C NMR. Results support Re5-PAN (φ = 0.021) functioning as a catalyst for the reduction of CO2 (maximum turn-over 48-50 at 300 equiv. triethylamine as the sacrificial electron donor).

Simulation and Assignment of the Terahertz Vibrational Spectra of Enalapril Maleate Cocrystal Polymorphs

The identification of crystalline drug polymorphs using terahertz vibrational spectroscopy is a powerful approach for the nondestructive and noninvasive characterization of solid-state pharmaceuticals. However, a complete understanding of the terahertz spectra of molecular solids is challenging to obtain because of the complex nature of the low-frequency vibrational motions found in the sub-3 THz (sub-100 cm^-1) range. Unambiguous assignments of the observed spectral features can be achieved through quantum mechanical solid-state simulations of crystal structures and lattice vibrations utilizing the periodic boundary condition approach. The terahertz spectra of two polymorphs of enalapril maleate are presented here to demonstrate that even large pharmaceuticals can be successfully modeled using solid-state density functional theory, including cocrystalline solids comprised of multiple distinct species. These simulations enable spectral assignments to be made, but also provide insights into the conformational and cohesion energies that contribute to the polymorph stabilities. The results reveal that the Form II polymorph of enalapril maleate is the more stable of the two under ambient conditions, and that this stability is driven by a greater intermolecular cohesion energy as compared to Form I.

Identifying and explaining vibrational modes of quinacridones via temperature-resolved terahertz spectroscopy: absorption experiments and solid-state density functional theory simulations

Quinacridone and its substituted analogs are pigments widely used in art and industry. The temperature dependence of the crystal structures of two quinacridone polymorphs (β and γ), along with the common variant 2,9-dimethylquinacridone, were investigated using powder X-ray diffraction and terahertz spectroscopy. These were then compared with solid-state density functional theory simulations of both structures and vibrations. X-ray patterns were collected at eight temperatures in the range 13-298 K and terahertz spectra at fifteen temperatures in the range 20-300 K. Simulations were at absolute zero and at appropriate expansions to model room temperature. It was found that some of the powder X-ray diffraction features in only β-quinacridone (15.7°, 19.7° and 31.2° at 13 K) underwent anomalous shifting with temperature change. We attribute this to the unique coplanar hydrogen bonding pattern of β-quinacridone compared to the other solids, with the unusual diffraction peaks originating from crystallographic planes perpendicular to the a axis intermolecular hydrogen bonds. This observation coincides with a contraction of the a axis with heating and results from its relatively weak N-HO hydrogen bonds and significant C-HH-C repulsions. Associated with this anomalous contraction, for β-quinacridone only spectral peaks are seen to increase in energy with temperature.

Mapping the polymorphic transformation gateway vibration in crystalline 1,2,4,5-tetrabromobenzene

A single lattice vibration at 15.5 cm^–1 serves as a gateway for the polymorphic conversion of thermosalient 1,2,4,5-tetrabromobenzene crystals.

The thermosalient behavior of 1,2,4,5-tetrabromobenzene (TBB) is related to a temperature-induced polymorphic structural change. The mechanism behind the phase transition has been investigated in this work using low-frequency (10–250 cm^–1) Raman spectroscopy and solid-state density functional theory simulations. Careful adjustments of the probing laser power permitted thermal control of the polymorph populations and enabled high-quality Raman vibrational spectra to be obtained for both the β (low temperature) and γ (high temperature) forms of TBB. Numerous well-defined vibrational features appear in the Raman spectra of both polymorphs which could be assigned to specific motions of the solid-state TBB molecules. It was discovered that the lowest-frequency vibration at 15.5 cm^–1 in β-TBB at 291 K is a rotational mode that functions as a gateway for inducing the polymorphic phase transition to γ-TBB, and serves as the initiating step in the storage of mechanical strain for subsequent macroscopic release. Computationally mapping the potential energy surface along this vibrational coordinate reveals that the two TBB polymorphs are separated by a 2.40 kJ mol^–1 barrier and that γ-TBB exhibits an enhanced cohesion energy that stabilizes its structure.

Optical Properties of Meloxicam in the Far-Infrared Spectral Region

One of the most commonly used nonsteroidal anti-inflammatory active pharmaceutical ingredient called Meloxicam has been characterized spectroscopically both by Terahertz (THz) time domain spectroscopy (THz-TDS) and by Fourier Transform Infrared (FTIR) spectroscopy in far-IR regions of electromagnetic spectrum; 0.2 THz to 20 THz. While many relatively sharp features are observed in the far-IR range between 2 THz to 20 THz as expected for being an organic substance, very distinct and relatively strong absorption bands are also observed at 1.00, 1.66, 2.07 and 2.57 THz in the THz range. These well separated, defined, and fairly strong spectral features can be used for discrimination and quantification of Meloxicam in drug analysis. Frequency dependent refractive index of the drug was determined in a range of 0.2 THz and 2.7 THz, where an almost constant index was observed with an average index of 1.75. Powder XRD, and solid-state Density Functional Theory (SS-DFT) calculations were utilized to determine the crystalline form of the Meloxicam sample in its enolic crystalline form. Single molecule DFT calculations were also performed in all four possible structures of Meloxicam. In addition, the capability of THz waves transmission through common packaging materials is demonstrated for possibility of future on-site analysis. The results suggest that drug analysis will be possible to perform not only at every stage of manufacturing without destruction but also directly at the shelf of a market after development of portable THz technologies.

Methyl-rotation dynamics in metal–organic frameworks probed with terahertz spectroscopy

In ZIF-8 and its cobalt analogue ZIF-67, the imidazolate methyl-groups, which point directly into the void space, have been shown to freely rotate – even down to cryogenic temperatures.

Comparison of Intermolecular Forces in Anhydrous Sorbitol and Solvent Cocrystals

The hygroscopicity of solid sorbitol is important for its utilization as a sweetener in the pharmaceutical and food industries. The molecular foundations of sorbitol hydration characteristics are explored here using two solvated cocrystals, sorbitol-water and sorbitol-pyridine. In this work, solid-state density functional theory and terahertz time-domain spectroscopy were used to evaluate the relative stabilities of these cocrystals as compared to anhydrous sorbitol in terms of conformational and cohesive energies. The modification of the hydrogen-bonding network in crystalline sorbitol by solvent molecules gives new insight into the origins of the notable stability of sorbitol-water as compared to similar solids such as mannitol-water. In particular, the energy analysis reveals that the relative instability of the mannitol hydrate is based primarily in the lack of water-water interactions which provide considerable stabilization in the sorbitol-water crystal.

Quantification of cation–anion interactions in crystalline monopotassium and monosodium glutamate salts

Specific anion–cation orbital interactions lead to the large structural and spectral differences observed in crystalline monosodium and monopotassium glutamates.

The significance of the amorphous potential energy landscape for dictating glassy dynamics and driving solid-state crystallisation

We show clear evidence for a theory proposing that the shape and structure of the PES is the fundamental factor underlying the dynamics at temperatures below the glass transition.

Concomitant polymorphism and the martensitic-like transformation of an organic crystal

Terahertz vibrational spectroscopy and solid-state density functional theory together reveal the true nature of a pseudo-continuous crystalline polymorphic phase transition.

Examination of l-Glutamic Acid Polymorphs by Solid-State Density Functional Theory and Terahertz Spectroscopy

The ability of l-glutamic acid to crystallize in two different forms has long been the subject of study due to its commercial importance. While a solvent-mediated phase transformation between the α and β polymorphs is the prevailing theory, recent reports indicate a thermal solid-solid transformation between the two may be possible. However, determining accurate thermodynamic stabilities of these crystals has been challenging. Here new low-temperature single-crystal X-ray diffraction data coupled to solid-state density functional theory simulations have enabled a detailed description to be achieved for the energetic parameters governing the stabilization of the two l-glutamic acid solids. The temperature-dependent Gibbs free-energy curves show that α-glutamic acid is the preferred form at low temperatures (<222 K) and the β form is most stable at ambient temperatures. Terahertz time-domain spectroscopy was utilized to evaluate the quality of the intermolecular force modeling as well as to provide characteristic low-frequency spectral data that can be used for quantification of polymorph mixtures or crystal growth monitoring.

Measuring the Elasticity of Poly-l-Proline Helices with Terahertz Spectroscopy

The rigidity of poly‐l‐proline is an important contributor to the stability of many protein secondary structures, where it has been shown to strongly influence bulk flexibility. The experimental Young's moduli of two known poly‐l‐proline helical forms, right‐handed all‐cis (Form I) and left‐handed all‐trans (Form II), were determined in the crystalline state by using an approach that combines terahertz time‐domain spectroscopy, X‐ray diffraction, and solid‐state density functional theory. Contrary to expectations, the helices were found to be considerably less rigid than many other natural and synthetic polymers, as well as differing greatly from each other, with Young's moduli of 4.9 and 9.6 GPa for Forms I and II, respectively.

Uncovering the Terahertz Spectrum of Copper Sulfate Pentahydrate

Terahertz vibrational spectroscopy has evolved into a powerful tool for the detection and characterization of transition metal sulfate compounds, specifically for its ability to differentiate between various hydrated forms with high specificity. Copper(II) sulfate is one such system where multiple crystalline hydrates have had their terahertz spectra fully assigned, and the unique spectral fingerprints of the forms allows for characterization of multicomponent systems with relative ease. Yet the most commonly occurring form, copper(II) sulfate pentahydrate (CuSO4·5H2O), has proven elusive due to the presence of a broad absorption across much of the terahertz region, making the unambiguous identification of its spectral signature difficult. Here, it is shown that the sub-100 cm(-1) spectrum of CuSO4·5H2O is obscured by absorption from adsorbed water and that controlled drying reveals sharp underlying features. The crystalline composition of the samples was monitored in parallel by X-ray diffraction as a function of drying time, supporting the spectroscopic results. Finally, the terahertz spectrum of CuSO4·5H2O was fully assigned using solid-state density functional theory simulations, helping attribute the additional absorptions that appear after excessive drying to formation of CuSO4·3H2O.

The crucial role of water in shaping low-barrier hydrogen bonds

Low-barrier hydrogen bonds (LBHBs) are key components in a range of chemical processes, often appearing in metal-mediated catalytic applications.

Origins of contrasting copper coordination geometries in crystalline copper sulfate pentahydrate

Metal-aqua ion ([M(H2O)n](X+)) formation is a fundamental step in mechanisms that are central to enzymatic and industrial catalysis. Past investigations of such ions have yielded a wealth of information regarding their properties, however questions still exist involving the exact structures of these complexes. A prominent example of this is hexaaqua copper(II) ([Cu(H2O)6](2+)), with the solution versus gas-phase configurations under debate. The differences are often attributed to the intermolecular interactions between the bulk solvent and the aquated complex, resulting in structures stabilized by extended hydrogen-bonding networks. Yet solution phase systems are difficult to study due to the lack of atomic-level positional details. Crystalline solids are ideal models for comparative study, as they contain fixed structures that can be fully characterized using diffraction techniques. Here, crystalline copper sulfate pentahydrate (CuSO4·5H2O), which contains two unique copper-water geometries, was studied in order to elucidate the origin of these contrasting hydrated metal envrionments. A combination of solid-state density functional theory and low-temperature X-ray diffraction was used to probe the electronic origins of this phenomenon. This was accomplished through implementation of crystal orbital overlap population and crystal orbital Hamiltonian population analyses into a developmental version of the CRYSTAL14 software. These new computational methods help highlight the delicate interplay between electronic structure and metal-water geometries.

The role of terahertz polariton absorption in the characterization of crystalline iron sulfate hydrates

Solid-state density functional theory indicates that polariton absorption plays a central role in understanding the identifying terahertz-frequency spectral features of hydrated iron sulfate compounds.

Terahertz disorder-localized rotational modes and lattice vibrational modes in the orientationally-disordered and ordered phases of camphor

Camphors' terahertz range spectra are measured and simulated, providing fundamental insight into the dynamics of its ordered and disordered phases.

Investigation of the terahertz vibrational modes of ZIF-8 and ZIF-90 with terahertz time-domain spectroscopy

We present experimental and computational evidence that gate-opening modes for zeolitic imidazole frameworks can be observed at terahertz frequencies.