Kinetics of Phase Transformations and of Other Time‐Dependent Processes in Solids Analyzed by Powder Diffraction

Leineweber, Andreas; Mittemeijer, E. J.

doi:10.1002/9783527649884.ch11

Cited by 4 publications

(2 citation statements)

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Section: Introductionmentioning

confidence: 99%

“…Recent advances in time-resolved synchrotron powder X-ray diffraction (sPXRD) enable the rapid collection of high resolution data under controlled environmental conditions. This makes it an ideal technique to study temporal structural changes associated with solid-state form changes, 12,13 though to date such methods have been applied to a relatively limited number of dehydration studies on minerals 14−17 and other molecular hydrates. 18−20 As one of the four DNA nucleobases and a component of several active pharmaceutical ingredients used in the treatment of diseases ranging from HIV to Hepatitis B, 21,22 investigations of thymine•••water interactions are arguably important.…”

Section: ■ Introductionmentioning

confidence: 99%

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Time-Resolved Cooperative Motions in the Solid-State Dehydration of Thymine Hydrate

Watts

Miehls

Swift

2020

Crystal Growth & Design

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Many organic compounds can exist in hydrated and anhydrous crystalline forms, each of which exhibits its own different set of physical properties. This makes dehydration–rehydration processes an important class of solid-state reactions; yet, the molecular-level mechanisms and cooperative motions which govern such transformation in molecular solids have been notoriously difficult to establish. Here using time-resolved synchrotron X-ray powder diffraction (sPXRD) and other methods, we identify a number of early subtle changes in thymine hydrate (TH) which set its trajectory for the formation of anhydrous products. An early cooperative “morning stretch” motion, characterized by a coordinated increase in the interlayer separation and angular rotation, was observed prior to the appearance of the first major anhydrous phase, Td1. Kinetic analyses indicated the overall solid-state reaction proceeds via a one-dimensional diffusion mechanism with an E a = 115–122 kJ/mol. At temperatures ≥45 °C, solid state dehydration yielded mixtures of Td1 and a second major anhydrous phase, Td2. Multiphase refinement of sPXRD data proved that Td2 is formed via two distinct routeseither directly due to water loss from TH or via the polymorphic transformation of Td1. Heating above ∼180 °C yielded Td2 as the major product. A limited number of weak diffraction peaks evidence the presence of a third transient form, Td*, which also converts to Td2. The time-resolved methods used here illustrate that solid-state dehydration even in a seemingly simple molecular hydrate system involves a significantly more complex set of coordinated molecular motions and solid–solid transformations than originally thought. The ability to glean a more complete picture of the cooperative motions that occur during water loss from soft crystalline hydrates is an important step in the development of a deeper understanding of this important class of materials.

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