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.

A new polymorphic quasiracemate d-2-aminobutyric acid:l-norvaline has been discovered and its reversible single-crystal-to-single-crystal phase transition has been studied using XRD, DSC and thermal microscopy. The low temperature form I is fully ordered, while form II shows two conformations for l-norvaline with a 50/50 occupancy.

The crystal structures of four di­meth­oxy­benzaldehyde (C9H10O3) isomers are reported and compared to the previously reported crystal structures of 3,4-di­meth­oxy­benzaldehyde and 2,6-di­meth­oxy­benzaldehyde. All di­meth­oxy­benzaldehyde mol­ecules in the crystal structure are nearly planar.