The crystal structure of p-aminobenzoic acid

A new insight into the mechanism of the tabletability flip phenomenon

Tabletability is an outcome of interparticulate bonding area (BA) - bonding strength (BS) interplay, influenced by the mechanical properties, size and shape, surface energetics of the constituent particles, and compaction parameters. Typically, a more plastic active pharmaceutical ingredient (API) exhibits a better tabletability than less plastic APIs due to the formation of a larger BA during tablet compression. Thus, solid forms of an API with greater plasticity are traditionally preferred if other critical pharmaceutical properties are comparable. However, the tabletability flip phenomenon (TFP) suggests that a solid form of an API with poorer tabletability may exhibit better tabletability when formulated with plastic excipients. In this study, we propose another possible mechanism of TFP, wherein softer excipient particles conform to the shape of harder API particles during compaction, leading to a larger BA under certain pressures and, hence, better tabletability. In this scenario, the BA-BS interplay is dominated by BA. Accordingly, TFP should tend to occur when API solid forms are formulated with a soft excipient. We tested this hypothesis by visualizing the deformation of particles in a model compressed tablet by nondestructive micro-computed tomography and by optical microscopy when the particles were separated from the tablet. The results confirmed that soft particles wrapped around hard particles at their interfaces, while an approximately flat contact was formed between two adjacent soft particles. In addition to the direct visual evidence, the BA-dominating mechanism was also supported by the observation that TFP occurred in the p-aminobenzoic acid polymorph system only when mixed with a soft excipient.

The ubiquity of the tabletability flip phenomenon

The plasticity of materials plays a critical role in adequate powder tabletability, which is required in developing a successful tablet product. Generally, a more plastic material can develop larger bonding areas when other factors are the same, leading to higher tabletability than less plastic materials. However, it was observed that, for a solid form of a compound with poorer tabletability, a mixture with microcrystalline cellulose (MCC) can actually exhibit better tabletability, a phenomenon termed tabletability flip. Hence, there is a chance that a solid form with poor tabletability could have been erroneously eliminated based on the expected tabletability challenges during tablet manufacturing. This study was conducted to investigate the generality of this phenomenon using two polymorph pairs, a salt and free acid pair, a crystalline and amorphous solid dispersion pair, and a pair of chemically distinct crystals. Results show that tabletability flip occurred in all six systems tested, including five pairs of binary mixtures with MCC and one pair in a realistic generic tablet formulation, suggesting the broad occurrence of the tabletability flip phenomenon, where both compaction pressure and the difference in plasticity between the pair of materials play important roles.

Different packing motifs in the crystal structures of three mol-ecular salts containing the 2-amino-5-carb-oxy-anilinium cation: C7H9N2O2 +·Cl-, C7H9N2O2 +·Br- and C7H9N2O2 +·NO3 -·H2O

The syntheses and crystal structures of three mol-ecular salts of protonated 3,4-di-amino-benzoic acid, viz. 2-amino-5-carb-oxy-anilinium chloride, C7H9N2O2 +·Cl-, (I), 2-amino-5-carb-oxy-anilinium bromide, C7H9N2O2 +·Br-, (II), and 2-amino-5-carb-oxy-anilinium nitrate monohydrate, C7H9N2O2 +·NO3 -·H2O, (III), are described. The cation is protonated at the meta-N atom (with respect to the carb-oxy group) in each case. In the crystal of (I), carb-oxy-lic acid inversion dimers linked by pairwise O-H⋯O hydrogen bonds are seen and each N-H group forms a hydrogen bond to a chloride ion to result in (100) undulating layers of chloride ions bridged by the inversion dimers into a three-dimensional network. The extended structure of (II) features O-H⋯Br, N-H⋯Br and N-H⋯O hydrogen bonds: the last of these generates C(7) chains of cations. Overall, the packing in (II) features undulating (100) sheets of bromide ions alternating with the organic cations. Inter-molecular inter-actions in the crystal of (III) include O-H⋯O, O-H⋯(O,O), N-H⋯O, N-H⋯N and O-H⋯N links. The cations are linked into (001) sheets, and the nitrate ions and water mol-ecules form undulating chains. Taken together, alternating (001) slabs of organic cations plus anions/water mol-ecules result. Hirshfeld surfaces and fingerprint plots were generated to give further insight into the inter-molecular inter-actions in these structures. The crystal used for the data collection of (II) was twinned by rotation about [100] in reciprocal space in a 0.4896 (15):0.5104 (15) ratio.

Polymorphism in p-aminobenzoic acid

We review the polymorphism of p-aminobenzoic acid (pABA), a model drug compound whose crystallisation and polymorphic behaviour has been extensively studied in recent years.

Conformational and structural stability of the single molecule and hydrogen bonded clusters of para aminobenzoic acid in the gas and solution phases

The structures of α- and β-para aminobenzoic acid are deconstructed into their hydrogen bonding molecular structural building blocks, where they are analysed usingab initioquantum mechanical calculations of their conformation and cluster stability in solution.

Aggregate Elasticity, Crystal Structure, and Tableting Performance for p-Aminobenzoic Acid and a Series of Its Benzoate Esters

The tableting performance for p-aminobenzoic acid (PABA) and a series of its benzoate esters with increasing alkyl chain length (methyl-, ethyl-, and n-butyl) was determined over a broad range of compaction pressures. The crystalline structure of methyl benzoate (Me-PABA) exhibits no slip systems and does not form viable compacts under any compaction pressure. The ethyl (Et-PABA) and n-butyl (Bu-PABA) esters each have a similar, corrugated-layer structure that displays a prominent slip plane and improves material plasticity at low compaction pressure. The compact tensile strength for Et-PABA is superior to that for Bu-PABA; however, neither material achieved a tensile strength greater than 2 MPa over the compression range studied. Complementary studies with powder Brillouin light scattering (BLS) show the maxima of the shear wave, acoustic frequency distribution red shift in an order consistent with both the observed tabletability and attachment energy calculations. Moreover, zero-porosity aggregate elastic moduli are determined for each material using the average acoustic frequency obtained from specific characteristics of the powder BLS spectra. The Young's moduli for Et- and Bu-PABA is significantly reduced relative to PABA and Me-PABA, and this reduction is further evident in tablet compressibility plots. PABA, however, is distinct with high elastic isotropy as interpreted from the narrow and well-defined powder BLS frequency distributions for both the shear and compressional acoustic modes. The acoustic isotropy is consistent with the quasi-isotropic distribution of hydrogen bonding for PABA. At low compaction pressure, PABA tablets display the lowest tensile strength of the series; however, above a compaction pressure of ca. 70 MPa PABA tablet tensile strength continues to increase while that for Et- and Bu-PABA plateaus. PABA displays lower plasticity relative to either ester, and this is consistent with its crystalline structure and high yield pressure determined from in-die Heckel analysis. Overall the complementary approach of using both structural and the acoustic inputs uniquely provided from powder BLS is anticipated to expand our comprehension of the structure-mechanics relationship and its role in tableting performance.

The influence of solution environment on the nucleation kinetics and crystallisability of para-aminobenzoic acid

The influence of solvent on the solution thermodynamics, nucleation-kinetics and crystal growth of alpha para-aminobenzoic acid (PABA) crystallising from supersaturated solutions, is examined through analysis of the metastable zone width.

Chemical Speciation and Bond Lengths of Organic Solutes by Core-Level Spectroscopy: pH and Solvent Influence on p-Aminobenzoic Acid

Through X-ray absorption and emission spectroscopies, the chemical, electronic and structural properties of organic species in solution can be observed. Near-edge X-ray absorption fine structure (NEXAFS) and resonant inelastic X-ray scattering (RIXS) measurements at the nitrogen K-edge of para-aminobenzoic acid reveal both pH- and solvent-dependent variations in the ionisation potential (IP), 1s→π* resonances and HOMO–LUMO gap. These changes unequivocally identify the chemical species (neutral, cationic or anionic) present in solution. It is shown how this incisive chemical state sensitivity is further enhanced by the possibility of quantitative bond length determination, based on the analysis of chemical shifts in IPs and σ* shape resonances in the NEXAFS spectra. This provides experimental access to detecting even minor variations in the molecular structure of solutes in solution, thereby providing an avenue to examining computational predictions of solute properties and solute–solvent interactions.

Towards an understanding of the nucleation of alpha-para amino benzoic acid from ethanolic solutions: a multi-scale approach

The molecular assembly and subsequent nucleation of para-amino benzoic acid (PABA) from ethanolic solutions is probed using a multi-scale and multi-technique approach. This is applied by examining and interrelating information regarding the molecular, solution-state, cluster, solid-state and surface structures to understand why the alpha form of PABA is crystallised in preference to its low temperature beta form. Calculations suggest that conformational changes within the solute molecule play little or no role in directing the nucleation of either the alpha or beta crystal forms. Combined ab initio and molecular dynamics calculations of the stability of small clusters in solution suggests that the hydrogen-bonded carboxylic acid dimers, present in the alpha structure, are the most stable in solution and play a major role in the self-assembly and polymorphic expression of the alpha form in ethanol in preference to the beta form. These calculations are in good agreement with X-ray small-angle scattering analysis which reveals the presence of PABA clusters in ethanol which are consistent with the size and shape of a carboxylic acid dimer. SAXS studies also reveal the presence of larger cluster structures in a size range 10-40 nm which appear to grow, perhaps reflecting a change in the balance between monomers and dimers within the solution during the nucleation process. The results of crystallisation-kinetics experiments indicate an instantaneous nucleation mechanism where the number of instantaneously nucleated crystallites is calculated to be 1360-660 nuclei per ml and the subsequent growth is found to be only rate limited by diffusion of the growth unit to the crystallite surface. A linear dependence of growth rate with respect to supersaturation is observed for the (0 1 -1) capping face, which is associated with strong π-π stacking interactions. This is consistent with a solid-on-solid mechanism associated with surface roughened growth and concomitant poor lattice-perfection. Conversely, the side (1 0 -1) surface has a growth mechanism consistent with a 2D nucleation birth and spread mechanism. Hence, these mechanisms result in very fast growth along the b-axis and the needle-like morphology that is observed for alpha-PABA.

Ostwald's rule and enantiotropy: polymorph appearance in the crystallisation of p-aminobenzoic acid

The development of rational crystallisation strategies in polymorphic systems requires the experimental manipulation of both kinetics and thermodynamics.

Incisive probing of intermolecular interactions in molecular crystals: core level spectroscopy combined with density functional theory

The α-form of crystalline para-aminobenzoic acid (PABA) has been examined as a model system for demonstrating how the core level spectroscopies X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine-structure (NEXAFS) can be combined with CASTEP density functional theory (DFT) to provide reliable modeling of intermolecular bonding in organic molecular crystals. Through its dependence on unoccupied valence states NEXAFS is an extremely sensitive probe of variations in intermolecular bonding. Prediction of NEXAFS spectra by CASTEP, in combination with core level shifts predicted by WIEN2K, reproduced experimentally observed data very well when all significant intermolecular interactions were correctly taken into account. CASTEP-predicted NEXAFS spectra for the crystalline state were compared with those for an isolated PABA monomer to examine the impact of intermolecular interactions and local environment in the solid state. The effects of the loss of hydrogen-bonding in carboxylic acid dimers and intermolecular hydrogen bonding between amino and carboxylic acid moieties are evident, with energy shifts and intensity variations of NEXAFS features arising from the associated differences in electronic structure and bonding.

A comparative study of two polymorphs of L-aspartic acid hydrochloride

Two polymorphs of L-aspartic acid hydrochloride, C4H8NO4(+)·Cl(-), were obtained from the same aqueous solution. Their crystal structures have been determined from single-crystal data collected at 100 K. The crystal structures revealed three- and two-dimensional hydrogen-bonding networks for the triclinic and orthorhombic polymorphs, respectively. The cations and anions are connected to one another via N-H···Cl and O-H···Cl interactions and form alternating cation-anion layer-like structures. The two polymorphs share common structural features; however, the conformations of the L-aspartate cations and the crystal packings are different. Furthermore, the molecular packing of the orthorhombic polymorph contains more interesting interactions which seems to be a favourable factor for more efficient charge transfer within the crystal.

The structural properties of a noncentrosymmetric polymorph of 4-aminobenzoic acid

The crystal structure of a polymorph of 4-aminobenzoic acid (PABA), C7H7NO2, at 100 K is noncentrosymmetric, as opposed to centrosymmetric in the structures of the other known polymorphs. The two crystallographically independent PABA molecules form pseudocentrosymmetric O-H...O hydrogen-bonded dimers that are further linked by N-H...O hydrogen bonds into a three-dimensional network. The benzene rings stack in the b direction. The CO2 moieties are bent out slightly from the benzene ring plane.

Hydrogen-bond coordination in organic crystal structures: statistics, predictions and applications

Statistical models to predict the number of hydrogen bonds that might be formed by any donor or acceptor atom in a crystal structure have been derived using organic structures in the Cambridge Structural Database. This hydrogen-bond coordination behaviour has been uniquely defined for more than 70 unique atom types, and has led to the development of a methodology to construct hypothetical hydrogen-bond arrangements. Comparing the constructed hydrogen-bond arrangements with known crystal structures shows promise in the assessment of structural stability, and some initial examples of industrially relevant polymorphs, co-crystals and hydrates are described.

p-Aminobenzoic acid polymorphs under high pressures

The effect of high pressure on two forms (α, β) of p-aminobenzoic acids (PABA) is studied in a diamond anvil cell using in situ Raman spectroscopy.

Anilinium-3-carboxyl-ate 3-carb-oxy-anilinium nitrate

The title compound, C7H8NO2(+)·NO3(-)·C7H7NO2, exists in the form of a protonated dimer of two anilinium-3-carboxyl-ate mol-ecules related by an inversion center, and a nitrate anion located on a twofold rotation axis. The bridging H atom occupies, with equal probability, the two sites associated with the carboxyl atoms. In addition to the strong O-H⋯O hydrogen bond, in the crystal, the various units are linked via N-H⋯O and C-H⋯O hydrogen bonds forming a three-dimensional structure.

Growth of oriented p-aminobenzoic acid crystals by directional freezing

Oriented long needle-like p-aminobenzoic acid (PABA) crystals are successfully prepared by directional freezing of PABA solution in this work. The width of the oriented crystals is controlled by changing the directional cooling rate, resulting in varying crystal morphologies and thermodynamic properties while maintaining the same chemical structure.

Structural, vibrational and quantum chemical investigations on 5-chloro-2-hydroxybenzamide and 5-chloro-2-hydroxybenzoic acid

The Fourier transform infrared (FTIR) and FT-Raman spectra of 5-chloro-2-hydroxybenzamide (5CBA) and 5-chloro-2-hydroxybenzoic acid (5C2HBA) have been recorded in the range 4000-400 and 4000-100 cm(-1), respectively. The complete vibrational fundamental modes of the compounds were assigned and analysed using the observed FTIR and FT-Raman data. The vibrational frequencies determined experimentally were compared with the theoretical wavenumbers calculated from ab initio HF and DFT-B3LYP gradient methods employing 6-31G** and 6-311++G** basis sets. The effect of halogen, hydroxyl groups and hydrogen bonding on the characteristic frequencies of the -COOH and -CONH2 group frequencies have been investigated. In 5CBA and 5C2HBA intramolecular hydrogen bond between a hydroxyl group and CO group makes a six membered ring, which causes the O⋯H interaction onto the resonance of the benzene ring. Comparison of the positions of the ν(OH) bands shows the ν(OH) band of 5CBA is located at considerably higher frequency which confirms a weaker hydrogen bond than in 5C2HBA.

FT-IR, NIR-FT-Raman and gas phase infrared spectra of 3-aminoacetophenone by density functional theory and ab initio Hartree-Fock calculations

The gas phase infrared spectrum of 3-aminoacetophenone (3AAP) was measured in the range 5000-500 cm(-1) and with a resolution of 0.5 cm(-1). The Fourier transform Raman (FT-Raman) and Fourier transform infrared (FT-IR) spectra of 3AAP were recorded in the solid phase. Geometry optimizations were done without any constraint and several thermodynamic parameters were calculated for the minimum energy conformer at ab initio and density functional theory (DFT) levels invoking 6-311G(2df 2p) basis set and the results are compared with the experimental values. Harmonic-vibrational wavenumber was also calculated for the minimum energy conformer at ab initio and DFT levels using 6-31G(d,p) basis set and the results are compared with related molecules. With the help of specific scaling procedures, the observed vibrational wavenumbers in gas phase, FT-IR and FT-Raman spectra were analyzed and assigned to different normal modes of the molecule. Most of the modes have wavenumbers in the expected range, the error obtained was in general very low. The appropriate theoretical spectrogram for the FT-IR spectra of the title molecule is also constructed.

Waste-free chemistry of diazonium salts and benign separation of coupling products in solid salt reactions

Gas-solid and solid-solid techniques allow for waste-free and quantitative syntheses in the chemistry of diazonium salts. Five techniques for diazotations with the reactive gases NO(2), NO and NOCl are studied. Two types are mechanistically investigated with atomic force microscopy (AFM) and are interpreted on the basis of known crystal packings. The same principles apply to the cascade reactions that had been derived from one-step reactions. Solid diazonium salts couple quantitatively with solid diphenylamine and anilines to give the triazenes. Azo couplings are achieved with quantitative yields by cautious co-grinding of solid diazonium salts with beta-naphthol and C-H acidic heterocycles, such as barbituric acids or pyrazolinones. Solid diazonium salts may be more easily applied in a stoichiometric ratio for couplings in solution. Co-grinding of solid diazonium salts with KI gives quantitative yields of various solid aryl iodides. The unavoidable coupling products in salt reactions are completely separated from the insoluble products in a highly benign manner. The solid-state reactions compare favourably with similar solution reactions that produce much waste. The structures of the products are elucidated with IR and NMR spectroscopy and mass spectrometry, while the tautomeric properties of the compounds are studied with density functional calculations at the B3LYP/6-31G* and BLYP/6-31G** levels.