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Is thermosalient effect possible without phase transition?

Oxitropium bromide and methylscopolamine bromide have very similar molecular structures, the only difference being that one ethyl group is replaced by methyl group in the case of methylscopolamine bromide. Both compounds have medical uses, oxitropium bromide is used as a bronchodilator, whereas methylscopolamine bromide used to prevent nausea and vomiting caused by motion sickness. They also both exhibit thermosalient effect – unexpected and abrupt jumping of the crystals during heating and cooling. This is where the difference stops. In the case of oxitropium bromide thermosalient effect is caused, as in most other thermosalient compounds, by the topotactic phase transition during which unit cell changes drastically thus causing the crystals to jump to heights several times larger than their dimensions. At the molecular level, the cation acts as a molecular shuttle composed of two rigid parts (epoxy-aza- tricyclic-nonyl portion and phenyl ring) that are bridged by a flexible ester linkage. The structure of the rigid, inert aza-tricyclic portion remains practically unaffected by the temperature, suggesting a mechanism in which the large, thermally accumulated strain is transferred over the ester bridge to the phenyl ring, which rotates to trigger the phase transition1. On the other hand, surprisingly, methylscopolamine bromide does not seem to show any phase transition at all, but yet, its crystals are also joyfully jumping around during the heating or cooling of sample. Both systems are characterized by uniaxial negative thermal expansion, but there is an abrupt change of the unit cell parameters during the phase transition for the oxitropium bromide whereas the parameters are changing perfectly linearly in the whole temperature range of existence for methylscopolamine bromide. Therefore, the question remains – what is causing the jumping in methylscopolamine bromide – a system so close to oxitropium bromide but exhibiting totally different kind of thermosalient effect.

Molecular crystals, jumping crystals, thermosalient effect, phase transition

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