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.

The Structural Pathway from its Solvated Molecular State to the Solution Crystallisation of the α- and β-Polymorphic Forms of Para Amino Benzoic Acid

Para amino benzoic acid has two well-characterised polymorphic forms and, whilst both crystallise in the monoclinic space group, they have quite different crystal chemistry and crystallisability behaviour. The molecular conformation...

Reactive crystallization: a review

Reactive crystallization is not new, but there has been recent growth in its use as a means of improving performance and sustainability of industrial processes.

Influence of solvent on crystal nucleation of benzocaine

The influence of the solvent in nucleation of benzocaine FII was explored by measuring nucleation induction times, probing solvent–solute interactions with spectroscopy and modelling the strength of solvent–solute intermolecular interactions using DFT.

Can solvated intermediates inform us about nucleation pathways? The case of β-pABA

Using crystallography to search for nucleation pathways: α and β polymorphs of p-aminobenzoic acid.

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.

Discovery and recovery of delta p-aminobenzoic acid

A new high-pressure recoverable form has been observed in the model system, p-aminobenzoic acid.

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.

Crystal nucleation and growth in a polymorphic system: Ostwald's rule, p-aminobenzoic acid and nucleation transition states

Water has a magic effect on the nucleation kinetics of an aminobenzoic acid – stacking interactions baffle Ostwald's rule and offer dual pathways.

Influence of Coformer Stoichiometric Ratio on Pharmaceutical Cocrystal Dissolution: Three Cocrystals of Carbamazepine/4-Aminobenzoic Acid

Cocrystallization is a technique to optimize solid forms that shows great potential to improve the solubility of active pharmaceutical ingredients (APIs). In some systems, an API can form cocrystals in multiple stoichiometries with the same coformer. However, it remains unclear how coformer stoichiometry influences solubility. This paper investigates the pharmaceutical:coformer pair carbamazepine (CBZ)/p-aminobenzoic acid (PABA); both CBZ/PABA 1:1 and 2:1 cocrystals are known, and a novel 4:1 CBZ/PABA cocrystal is reported here. The 4:1 cocrystal is structurally characterized, and phase stability data suggest that it is a thermodynamically unstable form. Dissolution experiments show that there is no correlation between the cocrystal stoichiometry and dissolution rate in this system. On the other hand, with the relatively weak intermolecular interactions, metastable forms can be beneficial to dissolution rate, which suggests that more effort should be devoted to cocrystal production with kinetic growth methods.

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.

Investigation into solid and solution properties of quinizarin

Polymorphism, crystal shape and solubility of 1,4-dihydroxyanthraquinone (quinizarin) have been investigated in acetic acid, acetone, acetonitrile, n-butanol and toluene.

Crystal nucleation of salicylic acid in organic solvents

Nucleation of salicylic acid in different solvents becomes gradually more difficult in the order: chloroform, ethyl acetate, acetonitrile, acetone, methanol and acetic acid. By comparing the results of metastable zone width and induction time experiments new insights are obtained.

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.