Investigation of the Low-Frequency Vibrations of Crystalline Tartaric Acid Using Terahertz Spectroscopy and Solid-State Density Functional Theory

On the use of a volume constraint to account for thermal expansion effects on the low-frequency vibrations of molecular crystals

A volume-constraint method is presented as a means to capture the influence of thermal expansion on the low-frequency vibrations in molecular crystals. In particular, the room-temperature terahertz absorption spectra of L-tartaric acid, α-lactose monohydrate, and α-para-aminobenzoic acid (PABA) have been simulated using dispersion-corrected, solid-state density functional theory (DFT-D). By comparing the normal modes obtained with a unit cell optimised without constraints to those obtained with a unit cell optimised while constrained to keep its experimental volume, wholesale improvements to the resultant spectrum is achieved when using the constrained geometry by inhibiting cell contraction. These improvements are demonstrated over a range of popular density functionals and basis sets up to triple-zeta complexity. A correlation method is then presented as a means to quantitatively compare the vibrational pattern of normal modes obtained from both unit cells. This analysis reveals that thermal expansion can effect the character and relative frequency of normal modes, with the choice of geometry ultimately affecting the assignment of the experimental absorptions. The sensibility of using the experimental volume as an approximation is then discussed, where it is speculated that large basis sets or hybrid functionals are necessary to ensure that the thermal expansion effect is not overestimated. The low-frequency absorption spectrum of PABA is then fully characterised using the PBE-D3BJ/6-311G(2d,2p) method.

Unveiling Five Naked Structures of Tartaric Acid

The unbiased, naked structures of tartaric acid, one of the most important organic compounds existing in nature and a candidate to be present in the interstellar medium, has been revealed in this work for the first time. Solid samples of its naturally occurring (R,R) enantiomer have been vaporized by laser ablation, expanded in a supersonic jet, and characterized by Fourier transform microwave spectroscopy. In the isolation conditions of the jet, we have discovered up to five different structures stabilized by intramolecular hydrogen-bond networks dominated by O-H⋅⋅⋅O=C and O-H⋅⋅⋅O motifs extended along the entire molecule. These five forms, two with an extended (trans) disposition of the carbon chain and three with a bent (gauche) disposition, can serve as a basis to represent the shape of tartaric acid. This work also reports the first set of spectroscopy data that can be used to detect tartaric acid in the interstellar medium.

Probing lattice vibration of alkali halide crystals by broadband terahertz spectroscopy

Terahertz spectral features of alkali halide crystals were studied with the combination of broadband terahertz time-domain spectroscopy and the solid-state-based density functional theory calculations. To understand the particular modes of the observed terahertz features of the alkali halide crystals, the resonant modes of KCl and CsCl were analyzed using face-centered cubic and body-centered cubic lattice models, respectively. The results show that the characteristic terahertz absorption peaks could be assigned to the lattice vibration of the ionic crystals. Furthermore, the terahertz responses of a series of alkali halides were recorded, and obvious absorption peaks were observed in each salt in the frequency region below 8.5 THz. What is more interestingly is that the frequencies of these observed peaks are red-shifted with the increases of the mass and radius of the ions. This correlation between the resonant frequency of the lattice vibration, the reduced atomic mass, and the equilibrium distance between the ions agrees well with the harmonic oscillator model.

Terahertz-spectroscopy for non-destructive determination of crystallinity of L-tartaric acid in smartFilms® and tablets made from paper

Newly developed active pharmaceutical ingredients (API) often experience low solubility in aqueous media and thus possess poor oral bioavailability. The SmartFilm®-technology is a novel approach to overcome poor solubility. The technique uses commercial paper in which API can be loaded in amorphous state, thus increasing dissolution rate dc/dt and solubility c_s when compared to bulk material. However, the preservation of the amorphous state is a prerequisite for an efficient use of the smartFilm-technology and thus the crystalline state needs to be inspected during storage. Preferably, this should be done non-destructively. Traditional techniques, such as x-ray diffraction (XRD) or differential scanning calorimetry (DSC), do not allow for non-destructive crystallinity investigations, whereas Terahertz (THz) spectroscopy is a non-destructive technique, that is sensitive to the crystalline state of many molecular crystals. Therefore, the potential of THz-spectroscopy for crystallinity state inspection of API in smartFilms and tablets made from smartFilms was investigated in this study. The THz results obtained were compared to results obtained from XRD and DSC measurements. Whereas DSC measurements failed to reliably detect crystalline API in the smartFilms, XRD and THz-spectroscopy showed similar results and revealed that it was possible to prepare smartFilms loaded with >23% (w/w) amorphous API. Results indicate the great potential of THz spectroscopy for the non-destructive determination of the crystalline state of APIs in smartFilms and/or tablets made from paper.

Terahertz spectroscopy of enantiomeric and racemic pyroglutamic acid

The low-frequency vibrational properties of D-, L- and DL-pyroglutamic acid (PGA) have been investigated with the terahertz time-domain spectroscopy (THz-TDS) from 0.5 to 4.5 THz. The enantiomers (D- and L-PGA) present similar absorption spectra, while the spectrum of racemate (DL-PGA) is obviously different. The temperature-dependent THz spectra of different PGA were recorded in the range of 293-83 K. The spectral changes during the cooling process suggest that D- and L-PGA undergo a structural phase transition, and no phase change of DL-PGA was found. The results indicate that THz spectroscopy is highly sensitive to the crystal structure of molecules. The density functional theory (DFT) calculations based on the crystal structures were performed to simulate the sample's THz spectra. It was demonstrated that the characteristic resonant absorption peaks of the enantiomeric and racemic PGA in the low-frequency THz region originate from the different vibrations, which corresponding to the specific structures and intermolecular interactions. The conformational diversity and fluctuation may help to understand the properties of PGA in biochemistry and functional material.

Structural insights into anhydrous and monohydrated forms of 2,4,6-trihydroxybenzoic acid based on Raman and terahertz spectroscopic characterization

In order to characterize molecular structures of 2,4,6-trihydroxybenzoic acid (PCA) by means of vibrational spectroscopic techniques, we report investigation of PCA monohydrated form and its anhydrous polymorphic one by using terahertz and Raman spectral characterization. The experimental THz spectra show that the monohydrated PCA only has two absorption bands at 0.69 and 1.65 THz respectively in the frequency region from 0.2 to 1.8 THz, meanwhile the anhydrous form has a few significantly different absorption bands at 0.75, 1.01, 1.46 and 1.64 THz, respectively. Furthermore, Raman spectra characterized such differences of vibrational modes shown within 200-1800 cm^-1 region about the monohydrated and anhydrous forms of PCA. In view of various possible theoretical structural forms that may exist in anhydrous PCA and its monohydrated one, density functional theory calculations were performed to simulate optimized structures and vibrational mode of above two PCA polymorphic forms. Theoretical results and experimental THz/Raman spectra of anhydrous PCA show that the dimer synthon via the carboxylic group ••• carboxyl group and its ortho-phenolic hydroxyl group inter-molecular hydrogen bonding interaction establishing the theoretical form I (AH-I) is more consistent with experimental observation than other theoretical forms (AH-II and AH-III). Meanwhile, the theoretical monohydrated form I (MH-I), which is formed by the linkage of carboxyl group and its ortho-phenolic hydroxyl group with water molecule, is also much more agreement with experimental spectral observations of PCA monohydrate than other monohydrated forms (MH-II and MH-III). Our study demonstrates effectively qualitative analysis of both micro-molecular structures and dehydrated transitions between anhydrous and hydrated polymorphic forms of PCA, thus providing rich information on the corresponding structural changes of anhydrous and hydrated PCAs due to various inter-molecular and intra-molecular interactions based on their finger-print vibrational spectra combined with theoretical simulations.

Structural investigation of a 2:1 co-crystal between diflunisal and isonicotinamide based on terahertz and Raman spectroscopy

In order to characterize molecular structure changes of drugs upon co-crystallization by means of spectroscopic techniques, vibrational spectra of solid-state diflunisal (DIF), isonicotinamide (ISO) and their 2:1 co-crystal have been investigated by using terahertz time-domain spectroscopy (THz-TDS) and Raman spectroscopy. A 2:1 DIF-ISO co-crystal between DIF and ISO has been synthesized by slow solution crystallization from ethanol. The experimental THz spectroscopy shows that the co-crystal has a few significantly different absorption peaks compared with raw parent materials within the frequency region from 0.2 to 1.6 THz. Likewise, some differences of vibrational spectra between the co-crystal and starting compounds could also be characterized by Raman spectral results. Density functional theory (DFT) was used to simulate optimized structures and vibrational modes of two kind of possible co-crystal theoretical forms (form I and II) between DIF and ISO. Theoretical co-crystal form I is shown with 2:1 theoretical binary-adduct formed by carboxylic acid-amide and carboxylic acid-pyridine under inter-molecular hydrogen bonding. Theoretical co-crystal form II has a similar structure as form I, meanwhile the only difference is that O63 atom simultaneously forms hydrogen bond with H33 and H64. Also the hydroxyl -OH and carboxyl group -COOH establish molecular heterocycle under intra-molecular hydrogen bonds in both forms. The theoretical results show that both THz and Raman spectra of co-crystal form II between DIF and ISO is more consistent with the experimental observations than those of co-crystal form I. These results provide us with a wealth of information and unique method for characterizing the composition of co-crystal structures and also inter-molecular hydrogen bonding interactions shown within pharmaceutical co-crystallization at the molecular level.

Terahertz Spectroscopy of Tetrameric Peptides

Determining the sequence and structure of peptides is crucial for understanding their structure-property relationships. Among many techniques, structures are typically elucidated using nuclear magnetic resonance spectroscopy and single crystal X-ray diffraction measurements. In this study, we present terahertz time-domain spectroscopy (THz-TDS) as a complementary, nondestructive technique that is sensitive to both the primary and secondary structures of tetrapeptides. Using only a few milligrams of peptide, THz-TDS spectra have been measured, some of which have been supported by density functional theory (DFT) calculations, to distinguish six tetrameric peptides with similar primary and secondary structures.

Experimental and theoretical investigations of tartaric acid isomers by terahertz spectroscopy and density functional theory

The terahertz (THz) absorption spectra of l-, d-, and dl-tartaric acid have been measured in the frequency range from 0.2 to 2.0 THz by terahertz time-domain spectroscopy (THz-TDS). The characteristic absorption peaks of these three tartaric acid isomers were obtained, which showed remarkable difference between enantiomers (l- and d-tartaric acid) and the racemic compound (dl-tartaric acid) in their peak frequencies. In parallel with the experimental study, theoretical calculations on isolated-molecule and unit cell of tartaric acids using density functional theory (DFT) were also performed for simulating the experimental THz spectrum features, which were in good agreement with the experimental data. Results demonstrate that THz-TDS can distinguish the tiny diversity between tartaric acid chiral isomers and its racemic compound, and provided an effective method for molecular identification in biological and biomedical engineering.

Terahertz spectroscopic identification of explosive and drug simulants concealed by various hiding techniques

Terahertz time-domain spectroscopy and imaging is used to study the effects of various hiding techniques of spectral features of drug and explosive simulants in combination with different paper and textile barriers. Results show that rapid detection and identification of concealed simulants is possible in the frequency range from 1.5 to 4.0 THz by using an organic-crystal-based terahertz time-domain system and the spectral peak analysis method.

Noncovalent interactions between modified cytosine and guanine DNA base pair mimics investigated by terahertz spectroscopy and solid-state density functional theory

Modified cytosine and guanine nucleobases cocrystallize in a hydrogen bonding configuration similar to that observed in native DNA. The noncovalent interactions binding these base pairs in the crystalline solid were investigated using terahertz (THz) spectroscopy and solid-state density functional theory (DFT). While stronger hydrogen bonding interactions are responsible for the general molecular orientations in the crystalline state, it is the weaker dipole-dipole and dispersion forces that determine the overall packing arrangement. The inclusion of dispersion interactions in the DFT calculations was found to be necessary to accurately simulate the unit cell structure and THz vibrational spectrum. Using properly modeled intermolecular potentials, the lattice vibrational motions of the cytosine and guanine derivatives were calculated. The vibrational characters of the modes exhibited by the DNA base pair mimic in the THz region were primarily rotational motions and are indicative of the energies and the nature of vibrations that would likely be observed between similar base pairs in DNA molecules.