Solid-state Density Functional Theory Research Articles

Identifying Ordered OH/F Anions in Hydroxyfluorides by a Terahertz Spectroscopic Approach

Distinguishing the OH–/F– ligands has been one of the vital challenges in X-ray crystallography. Fluorine and oxygen atoms have similar scattering cross sections of X-rays, and the hydrogen atoms are invisible to X-rays, rendering the two functional groups F– and OH– indistinguishable from each other with the X-ray diffraction (XRD) method. We demonstrated that terahertz (THz) spectroscopy is promising for solving this problem. THz spectroscopy probes the optical phonon modes given rise by intermolecular interactions. Different F–/OH– substitutions usually lead to remarkable changes in the intermolecular interactions, which can be reflected in THz spectroscopy. Guanidinium fluorooxoborates represent new-generation nonlinear optical materials used in the short-wavelength ultraviolet region. For such materials, a dispute remains regarding the OH–/F– substitution in the B3O3(F, OH)4 anion units, which suggests 16 possible configurations of the anion units. Employing solid-state density functional theory, we showed that all 16 configurations display fingerprint THz peaks. Against the experimental criterion of high-resolution THz spectroscopy, we proved that only one configuration exists, implying that the anion structure is well-ordered. THz spectroscopy provides an essential complementary tool to the XRD method for retrieving detailed structural information. Furthermore, the terahertz vibrational information reveals that the cationic unit of guanidinium is a rigid building block unit with little external and internal vibration mixing, whereas the anionic unit is not rigid and produces vigorous mixing. This finding leads to a deeper understanding of the role of the constituent units in producing the excellent optical properties of the guanidinium fluorooxoborate compounds.

Direct Transformation of SiH4 to a Molecular L(H)2Co═Si═Co(H)2L Silicide Complex.

The synthesis of bimetallic molecular silicide complexes is reported, based on the use of multiple Si-H bond activations in SiH4 at the metal centers of 14-electron LCoI fragments (L = Tp″, HB(3,5-diisopropylpyrazolyl)3-; [BP2tBuPz], PhB(CH2PtBu2)2(pyrazolyl)). Upon exposure of (Tp″Co)2(μ-N2) (1) to SiH4, a mixture of (Tp″Co)2(μ-H) (2) and (Tp″Co)2(μ-H)2 (3) was formed and no evidence for Si-H oxidative addition products was observed. In contrast, [BP2tBuPz]-supported Co complexes led to Si-H oxidative additions with the generation of silylene and silicide complexes as products. Notably, the reaction of ([BP2tBuPz]Co)2(μ-N2) (5) with SiH4 gave the dicobalt silicide complex [BP2tBuPz](H)2Co═Si═Co(H)2[BP2tBuPz] (8) in high yield, representing the first direct route to a symmetrical bimetallic silicide. The effect of the [BP2tBuPz] ligand on Co-Si bonding in 7 and 8 was explored by analysis of solid-state molecular structures and density functional theory (DFT) investigations. Upon exposure to CO or DMAP (DMAP = 4-dimethylaminopyridine), 8 converted to the corresponding [BP2tBuPz]Co(L)x adducts (L = CO, x = 2; L = DMAP, x = 1) with concomitant loss of SiH4, despite the lack of significant Si-H interactions in the starting complex. On heating to 60 °C, 8 underwent reaction with MeCl to produce small quantities of MexSiH4-x (x = 1-3), demonstrating functionalization of the μ-silicon atom in a molecular silicide to form organosilanes.

Gas-phase and solid-state electronic structure analysis and DFT benchmarking of HfCO.

Ab initio multi-reference configuration interaction (MRCI) and coupled cluster singles doubles and perturbative triples [CCSD(T)] levels of theory were used to study ground and excited electronic states of HfCO. We report potential energy curves, dissociation energies (De), excitation energies, harmonic vibrational frequencies, and chemical bonding patterns of HfCO. The 3Σ- ground state of HfCO has an 1σ22σ21π2 electron configuration and a ∼30 kcal mol-1 dissociation energy with respect to its lowest-energy fragments Hf(3F) + CO(X1Σ+). We further evaluated the De of its isovalent HfCX (X = S, Se, Te, Po) series and observed that they increase linearly from the lighter HfCO to the heavier HfCPo with the dipole moment of the CX ligand. The same linear relationship was observed for TiCX and ZrCX. We utilized the CCSD(T) benchmark values of De, excitation energy, and ionization energy (IE) values to evaluate density functional theory (DFT) errors with 23 exchange-correlation functionals spanning GGA, meta-GGA, global GGA hybrid, meta-GGA hybrid, range-separated hybrid, and double-hybrid functional families. The global GGA hybrid B3LYP and range-separated hybrid ωB97X performed well at representing the ground state properties of HfCO (i.e., De and IE). Finally, we extended our DFT analysis to the interaction of a CO molecule with a Hf surface and observed that the surface chemisorption energy and the gas-phase molecular dissociation energy are very similar for some DFAs but not others, suggesting moderate transferability of the benchmarks on these molecules to the solid state.

Deciphering the potassium storage phase conversion mechanism of phosphorus by combined solid-state NMR spectroscopy and density functional theory calculations

Phosphorus is the potential anode material for emerging potassium-ion batteries (PIBs) owing to the highest specific capacity and relatively low operation plateau. However, the reversible delivered capacities of phosphorus-based anodes, in reality, are far from the theoretical capacity corresponding to the formation of K3P alloy. And, their underlying potassium storage mechanisms remain poorly understood. To address this issue, for the first time, we perform high-resolution solid-state 31P NMR combined with XRD measurements, and density functional theory calculations to yield a systemic quantitative understanding of (de)potassiation reaction mechanism of phosphorus anode. We explicitly reveal a previously unknown asymmetrical nanocrystalline-to-amorphous transition process via rP ↔ (K3P11, K3P7, beta-K4P6) ↔ (alpha-K4P6) ↔ (K1−xP, KP, K4−xP3, K1+xP) ↔ (amorphous K4P3, amorphous K3P) that are proceed along with the electrochemical potassiation/depotassiation processes. Additionally, the corresponding K-P alloys intermediates, such as the amorphous phases of K4P3, K3P, and the nonstoichiometric phases of “K1−xP”, “K1+xP”, “K4−xP3” are experimentally detected, which indicating various complicated K-P alloy species are coexisted and evolved with the sluggish electrochemical reaction kinetics, resulting in lower capacity of phosphorus-based anodes. Our findings offer some insights into the specific multi-phase evolution mechanism of alloying anodes that may be generally involved in conversion-type electrode materials for PIBs.

Terahertz spectroscopy for quantitatively elucidating the crystal transformation of chiral histidine enantiomers to racemic compounds

d-Histidine (d-His), l-Histidine (l-His), and their racemic compound dl-Histidine (dl-His) have different stereo chirality, making them intrinsic diverse functionalities to the living system. Identifying the configuration and crystal structures of enantiomers and the racemic compound is always the foremost requirement in processing protein foods. Although these features can be analyzed by spectroscopic technologies individually, it remains challenging to incorporate these complex methods into a facile analytical strategy. Herein, we propose a terahertz spectroscopy with solid-state density functional theory to both distinguish the configurational difference and quantify the crystal transformation from l-His and d-His to dl-His. By comparison with X-ray diffraction analysis, the validity of the crystal transformation evaluation based on terahertz spectroscopy is verified. A normalized fitting line regarding the terahertz absorption frequency and intensity is calculated to quantitatively elucidate the crystal transformation from enantiomers to dl-His. Our findings provide a new analytical approach to the research on food chemistry.

Constructing robust cathode/Li interfaces and intensifying ion transport kinetics for PEO-based solid-state lithium-sulfur batteries

Solid-state lithium-sulfur batteries (SSLSBs) are attractive due to their potential to provide high energy densities. However, the battery chemistry of SSLSBs still faces significant challenges, such as sluggish ion transport and prominent interface failures. Here, a high-performance cell is demonstrated by designing a composite electrolyte (CPE) consisting of polyethylene glycol azobenzamide (PGA) and poly (ethylene oxide) (PEO). Meanwhile, 7Li solid-state NMR spectroscopy and density functional theory (DFT) results show that PGA promotes the dissociation of lithium salts and generate more free lithium ions, thereby improving the ion migration kinetics. Moreover, ex-situ XPS and DFT confirm that PGA has a significant confined effect on polysulfides and effectively suppresses the shuttle effect. Subsequently, the interfacial layer consisted of abundant LiF&Li3N, which could achieve rapid lithium-ion transport and uniform deposition. Therefore, a stable ultra-long cycle time of over 2800 h can be achieved for the Li/Li symmetric cells at 0.2 mA cm−2. The prepared CPE exhibits a high ionic conductivity of 4.9 × 10-4 S cm−1 and a lithium-ion transference number of 0.53 at 50 °C. The assembled SSLSBs show an excellent cycling performance, which accomplishes 100 cycles at 0.2C with the reversible discharge capacity of 541 mAh/g. The superior performance of the SSLSBs rationalizes the CPE construction concept and leads to considerations for the innovative design of PEO-based SSLSBs.

Hydrates of active pharmaceutical ingredients: A 35Cl and 2H solid-state NMR and DFT study

This study uses 35Cl and 2H solid-state NMR (SSNMR) spectroscopy and dispersion-corrected plane-wave density functional theory (DFT) calculations to characterize the molecular-level structures and dynamics of hydrates of active pharmaceutical ingredients (APIs). We use 35Cl SSNMR to measure the EFG tensors of the chloride ions to characterize hydrated forms of hydrochloride salts of APIs, along with two corresponding anhydrous forms. DFT calculations are used to refine the crystal structures of the APIs and determine relationships between the 35Cl EFG tensors and the spatial arrangements of proximate hydrogen bonds, which are particularly influenced by interactions with water molecules. We find that the relationship between 35Cl EFG tensors and local hydrogen bonding geometries is complex, but meaningful structure/property relationships can be garnered through use of DFT calculations. Specifically, for every case in which such a comparison could be made, we find that the hydrate has a smaller magnitude of CQ than the corresponding anhydrous form, indicating a chloride ion environment with a ground-state electron density of higher spherical symmetry in the former. Finally, variable-temperature 35Cl and 2H SSNMR experiments on a deuterium-exchanged sample of the API cimetidine hydrochloride monohydrate are used to monitor temperature-dependent influences on the spectra that may arise from motional influences on the 35Cl and 2H EFG tensors. From the 2H SSNMR spectra, we determine that the motions of water molecules are characterized by jump-like motions about their C2 rotational axes that occur on timescales that are unlikely to influence the 35Cl central-transition (+1/2 ↔︎ −1/2) powder patterns (this is confirmed by 35Cl SSNMR). Together, these methods show great promise for the future study of APIs in their bulk and dosage forms, especially variable hydrates in which crystallographic water content varies with external conditions such as humidity.

An investigation on 3-hydroxybenzaldehyde for exploring terahertz low-frequency vibration modes with quasi-harmonic approximation method

3-Hydroxybenzaldehyde (3-HBA) was investigated in the range of 0.6–2.8 THz by terahertz time-domain spectroscopy (THz-TDS) and solid-state density functional theory (ss-DFT) with first-principles calculation. Four distinct peaks were found respectively, and among them, the intensity disparity between experiment and simulation spectra at 2.04 THz was recognized as the biggest inconsistency. Considering thermal behavior can be responsible for this, quasi-harmonic approximation (QHA) method was introduced to mimic the unit cell volume expansion. According to vibrational modes analysis, it was ascertained that the biggest vibrational modes discrepancy was also located at 2.04 THz. Molecules in 0% and 4% unit cell expansion exhibit an opposite rotational direction in a-b plane compared with 2% unit cell expansion. Noncovalent intermolecular interactions were investigated with independent gradient model (IGM), and the result indicates that hydrogen bonding is the dominating noncovalent interaction of 3-HBA. While calculating systematic potential energy to the displaced bonds stretching involving hydrogen atoms, it was found the anomalous potential energy variation to the bond stretching provides a possible explanation for the rotation direction divergence, that is, the rotation direction divergence can be related to some hydrogen atoms seeking lower overall potential energy around their equilibrium positions during bond stretching in response to the variational intermolecular van der Waals force. This research combined THz-TDS with the quasi-harmonic approximation method, elucidating the principle of vibrational characteristics in different volumes, which is beneficial to the investigation of the terahertz low-frequency vibration to thermal behavior as a reference in biochemistry and other fields.

Evaluation of the Stability of Diamine-Appended Mg2(dobpdc) Frameworks to Sulfur Dioxide.

Diamine-appended Mg2(dobpdc) (dobpdc4- = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate) metal-organic frameworks are a promising class of CO2 adsorbents, although their stability to SO2─a trace component of industrially relevant exhaust streams─remains largely untested. Here, we investigate the impact of SO2 on the stability and CO2 capture performance of dmpn-Mg2(dobpdc) (dmpn = 2,2-dimethyl-1,3-propanediamine), a candidate material for carbon capture from coal flue gas. Using SO2 breakthrough experiments and CO2 isobar measurements, we find that the material retains 91% of its CO2 capacity after saturation with a wet simulated flue gas containing representative levels of CO2 and SO2, highlighting the robustness of this framework to SO2 under realistic CO2 capture conditions. Initial SO2 cycling experiments suggest dmpn-Mg2(dobpdc) may achieve a stable operating capacity in the presence of SO2 after initial passivation. Evaluation of several other diamine-Mg2(dobpdc) variants reveals that those with primary,primary (1°,1°) diamines, including dmpn-Mg2(dobpdc), are more robust to humid SO2 than those featuring primary,secondary (1°,2°) or primary,tertiary (1°,3°) diamines. Based on the solid-state 15N NMR spectra and density functional theory calculations, we find that under humid conditions, SO2 reacts with the metal-bound primary amine in 1°,2° and 1°,3° diamine-appended Mg2(dobpdc) to form a metal-bound bisulfite species that is charge balanced by a primary ammonium cation, thereby facilitating material degradation. In contrast, humid SO2 reacts with the free end of 1°,1° diamines to form ammonium bisulfite, leaving the metal-diamine bond intact. This structure-property relationship can be used to guide further optimization of these materials for CO2 capture applications.

First-principles study of elastic and thermodynamic properties of UO2, γ-UO3 and α-U3O8

A detailed solid-state density functional theory study of uranium dioxide (UO2), gamma uranium trioxide (γ-UO3), and alpha triuranium octoxide (α-U3O8), which are very common materials throughout the nuclear fuel cycle, was carried out. DFT simulations of these three oxides were presented using both PBE+U and LDA+U functionals to examine their mechanical and thermodynamic properties. The computed lattice parameters were found to be in very good agreement with previous experimental results. The values of elastic properties like bulk, shear, and Young's modulus, as well as Poisson's ratio, were predicted, and the calculated Debye temperature was reported. The elastic anisotropy of UO2, γ-UO3, and α-U3O8 was calculated for the first time, from which the anisotropic degrees of these oxides were revealed by the values of anisotropic parameters and three-dimensional surface constructions of elastic properties. In addition, thermodynamic properties of UO2, γ-UO3, and α-U3O8, including heat capacity, entropy, enthalpy, phonon free energy, and Gibbs free energy, were determined as a function of temperature in the range of 0 - 2000 K. The computed thermodynamic values show an overall good agreement with available experiments, especially at low temperatures. The obtained physical parameters can provide meaningful data for further experimental investigations and promote the development of nuclear fuel cycles.

Tunable Solid Acid Catalyst Thin Films Prepared by Atomic Layer Deposition.

Solid acid catalysts, including zeolites and amorphous silica-aluminas (ASAs), are industrially important materials widely used in the fuel and petrochemical industries. The versatility of zeolites is due to the Brønsted acidity of the bridging hydroxyl and shape selectivity that can be tailored during and after synthesis. This is in contrast to amorphous silica-alumina, where tailoring acidity is a major challenge as the Brønsted acid structure in ASA is still debated. In both cases, however, the pore size and acidity cannot be tuned independently, and this is particularly limiting in the application of biomass conversion, where zeolite pores are too small for the molecules of interest. Herein, we present a method using atomic layer deposition (ALD) to prepare thin films of solid acid materials where the ratio of Brønsted to Lewis acid sites can be tuned precisely. This capability, combined with the sub-nm pore size control afforded by ALD yields a powerful and flexible method for synthesizing solid acid catalysts inside virtually any mesoporous host. We demonstrate the utility of these materials in two acid-catalyzed reactions relevant to biomass conversion: (1) Meerwein-Ponndorf-Verley-Oppenauer (MPVO) reaction and dehydration of fructose and (2) cascade reaction of glucose to 5-hydroxymethylfurfural. Finally, we propose a plausible structure for the Brønsted acid sites in our materials based on infrared spectroscopy and solid-state nuclear magnetic resonance measurements and density functional theory calculations and argue that this same structure might apply to conventional ASAs as well.

Increased CO2 Affinity and Adsorption Selectivity in MOF-801 Fluorinated Analogues.

The novel ZrIV-based perfluorinated metal–organic framework (PF-MOF) [Zr6O4(OH)4(TFS)6] (ZrTFS) was prepared under solvent-free conditions using the commercially available tetrafluorosuccinic acid (H2TFS) as a bridging ditopic linker. Since H2TFS can be seen as the fully aliphatic and perfluorinated C4 analogue of fumaric acid, ZrTFS was found to be isoreticular to zirconium fumarate (MOF-801). The structure of ZrTFS was solved and refined from X-ray powder diffraction data. Despite this analogy, the gas adsorption capacity of ZrTFS is much lower than that of MOF-801; in the former, the presence of bulky fluorine atoms causes a considerable window size reduction. To have PF-MOFs with more accessible porosity, postsynthetic exchange (PSE) reactions on (defective) MOF-801 suspended in H2TFS aqueous solutions were carried out. Despite the different H2TFS concentrations used in the PSE process, the exchanges yielded two mixed-linker materials of similar minimal formulae [Zr6O4(μ3-OH)4(μ1-OH)2.08(H2O)2.08(FUM)4.04(HTFS)1.84] (PF-MOF1) and [Zr6O4(μ3-OH)4(μ1-OH)1.83(H2O)1.83(FUM)4.04(HTFS)2.09] (PF-MOF2) (FUM2– = fumarate), where the perfluorinated linker was found to fully replace the capping acetate in the defective sites of pristine MOF-801. CO2 and N2 adsorption isotherms collected on all samples reveal that both CO2 thermodynamic affinity (isosteric heat of adsorption at zero coverage, Qst) and CO2/N2 adsorption selectivity increase with the amount of incorporated TFS2–, reaching the maximum values of 30 kJ mol–1 and 41 (IAST), respectively, in PF-MOF2. This confirms the beneficial effect coming from the introduction of fluorinated linkers in MOFs on their CO2 adsorption ability. Finally, solid-state density functional theory calculations were carried out to cast light on the structural features and on the thermodynamics of CO2 adsorption in MOF-801 and ZrTFS. Due to the difficulties in modeling a defective MOF, an intermediate structure containing both linkers in the framework was also designed. In this structure, the preferential CO2 adsorption site is the tetrahedral pore in the “UiO-66-like” structure. The extra energy stabilization stems from a hydrogen bond interaction between CO2 and a hydroxyl group on the inorganic cluster.

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.

Anisotropic Band-Edge Absorption of Millimeter-Sized Zn(3-ptz)2 Single-Crystal Metal-Organic Frameworks.

Metal–organic frameworks (MOFs) have emerged as promising tailor-designed materials for developing next-generation solid-state devices with applications in linear and nonlinear coherent optics. However, the implementation of functional devices is challenged by the notoriously difficult process of growing large MOF single crystals of high optical quality. By controlling the solvothermal synthesis conditions, we succeeded in producing large individual single crystals of the noncentrosymmetric MOF Zn(3-ptz)2 (MIRO-101) with a deformed octahedron habit and surface areas of up to 37 mm2. We measured the UV–vis absorption spectrum of individual Zn(3-ptz)2 single crystals across different lateral incidence planes. Millimeter-sized single crystals have a band gap of Eg = 3.32 eV and exhibit anisotropic absorption in the band-edge region near 350 nm, whereas polycrystalline samples are fully transparent in the same frequency range. Using solid-state density functional theory (DFT), the observed size dependence in the optical anisotropy is correlated with the preferred orientation adopted by pyridyl groups under conditions of slow crystal self-assembly. Our work thus paves the way for the development of optical polarization switches based on metal–organic frameworks.

Terahertz spectroscopy and lattice vibrational analysis of pararealgar and orpiment

Terahertz time-domain spectroscopy (THz-TDS) is an effective nondestructive and noninvasive tool for investigating sulfur-containing pigments. Combined with Raman spectroscopy and vibrational mode analysis, it is significant for artifact identification and conservation. In this work, the terahertz absorption spectra of pararealgar (As4S4) and orpiment (As2S3) samples mixed with polytetrafluoroethylene (PTFE) are characterized in a range from 0.2 THz to 2.2 THz, and their distinctive peaks are observed, respectively. Meanwhile, qualitative analysis is also implemented by using Raman spectroscopy as a complementary technique. The lattice vibrations are simulated by using solid-state density functional theory (ss-DFT), illustrating different characteristic absorption peaks for specific crystalline structures and dynamic properties. This work provides a reliable database of sulfur-containing pigments for using the THz technology to actually analyze and diagnose cultural relics.

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.

Characterization of Chemisorbed Species and Active Adsorption Sites in Mg-Al Mixed Metal Oxides for High-Temperature CO2 Capture.

Mg–Al mixed metal oxides (MMOs), derived from the decomposition of layered double hydroxides (LDHs), have been purposed as adsorbents for CO2 capture of industrial plant emissions. To aid in the design and optimization of these materials for CO2 capture at 200 °C, we have used a combination of solid-state nuclear magnetic resonance (ssNMR) and density functional theory (DFT) to characterize the CO2 gas sorption products and determine the various sorption sites in Mg–Al MMOs. A comparison of the DFT cluster calculations with the observed 13C chemical shifts of the chemisorbed products indicates that mono- and bidentate carbonates are formed at the Mg–O sites with adjacent Al substitution of an Mg atom, while the bicarbonates are formed at Mg–OH sites without adjacent Al substitution. Quantitative 13C NMR shows an increase in the relative amount of strongly basic sites, where the monodentate carbonate product is formed, with increasing Al/Mg molar ratios in the MMOs. This detailed understanding of the various basic Mg–O sites presented in MMOs and the formation of the carbonate, bidentate carbonate, and bicarbonate chemisorbed species yields new insights into the mechanism of CO2 adsorption at 200 °C, which can further aid in the design and capture capacity optimization of the materials.

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.

Terahertz spectral properties of glucose and two disaccharides in solid and liquid states

SummaryThe vibrational and rotational frequencies of most biological macromolecules fall within the terahertz (THz) band; therefore, the THz wave has a strong ability to distinguish substances. Saccharides are important organic substances and the main source of life-sustaining activities. In this study, the spectral characteristics of D-glucose, α-lactose hydrate, and β-maltose hydrate were measured in the solid state through THz time-domain spectroscopy in the frequency range of 0.1–2.5 THz. The crystal configurations of these three saccharides were then simulated using solid-state density functional theory, and the experimental results were found to be in good agreement with the simulation results. Furthermore, the spectral characteristics of the three saccharides in solutions were measured. Each saccharide was found to have unique spectral characteristics, and a correlation existed between the THz absorption spectra of the same substance in the solid state and aqueous solution.

From Operando Raman Mechanochemistry to "NMR Crystallography": Understanding the Structures and Interconversion of Zn-Terephthalate Networks Using Selective 17O-Labeling.

The description of the formation, structure, and reactivity of coordination networks and metal–organic frameworks (MOFs) remains a real challenge in a number of cases. This is notably true for compounds composed of Zn2+ ions and terephthalate ligands (benzene-1,4-dicarboxylate, BDC) because of the difficulties in isolating them as pure phases and/or because of the presence of structural defects. Here, using mechanochemistry in combination with operando Raman spectroscopy, the observation of the formation of various zinc terephthalate compounds was rendered possible, allowing the distinction and isolation of three intermediates during the ball-milling synthesis of Zn3(OH)4(BDC). An “NMR crystallography” approach was then used, combining solid-state NMR (1H, 13C, and 17O) and density functional theory (DFT) calculations to refine the poorly described crystallographic structures of these phases. Particularly noteworthy are the high-resolution 17O NMR analyses, which were made possible in a highly efficient and cost-effective way, thanks to the selective 17O-enrichment of either hydroxyl or terephthalate groups by ball-milling. This allowed the presence of defect sites to be identified for the first time in one of the phases, and the nature of the H-bonding network of the hydroxyls to be established in another. Lastly, the possibility of using deuterated precursors (e.g., D2O and d4-BDC) during ball-milling is also introduced as a means for observing specific transformations during operando Raman spectroscopy studies, which would not have been possible with hydrogenated equivalents. Overall, the synthetic and spectroscopic approaches developed herein are expected to push forward the understanding of the structure and reactivity of other complex coordination networks and MOFs.