Abstract
During single-crystal-to-single-crystal (SCSC) phase transitions, a polymorph of a compound can transform to a more stable form while remaining in the solid state. By understanding the mechanism of these transitions, strategies can be developed to control this phenomenon. This is particularly important in the pharmaceutical industry, but also relevant for other industries such as the food and agrochemical industries. Although extensive literature exists on SCSC phase transitions in inorganic crystals, it is unclear whether their classications and mechanisms translate to molecular crystals, with weaker interactions and more steric hindrance. A comparitive study of SCSC phase transitions in aliphatic linear-chain amino acid crystals, both racemates and quasi-racemates, is presented. A total of 34 transitions are considered and most are classified according to their structural change during the transition. Transitions without torsional changes show very different characteristics, such as transition temperature, enthalpy and free energy, compared with transitions that involve torsional changes. These differences can be rationalized using classical nucleation theory and in terms of a difference in mechanism; torsional changes occur in a molecule-by-molecule fashion, whereas transitions without torsional changes involve cooperative motion with multiple molecules at the same time.
Highlights
Polymorphism is the ability of a compound to exist in more than one crystal structure form, a very common phenomenon (Bernstein, 2002)
We show that we can distinguish several types of phase transition with different characteristics in racemates and quasi-racemates of linear-chain amino acids
Transitions involving torsional changes proceed through a ‘classical first-order transition’, with small hysteresis, large enthalpy differences and a clear nucleation-and-growth mechanism
Summary
Polymorphism is the ability of a compound to exist in more than one crystal structure form, a very common phenomenon (Bernstein, 2002). The study of phase transitions in molecular crystals has recently gained a renewed interest after the report of several examples of dynamic crystals These crystalline materials respond to external stimuli (e.g. exposure to light, pressure or temperature) with mechanical motion such as translation, rotation, jumping, bending and twisting (Naumov et al, 2015; Commins et al, 2016; Reddy et al, 2010). We compare solid-state phase transitions in several aliphatic linear-chain amino acids (see Fig. 1), the racemates dl-2-aminobutyric acid (dl-Abu), dl-norvaline (dl-Nva), dl-norleucine (dl-Nle), dl-2-aminoheptanoic acid (dl-Hep), dl-2-aminooctanoic acid (dl-Oct), dl-methionine (dl-Met) and their mutual quasi-racemates These amino acids all crystallize in similar packings and all exhibit SCSC phase transitions. Growth theory and cooperative motion are compatible with our results, and are not mutually exclusive
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