engleski

Can we predict the jumping of crystals? The case of oxitropium salts.

Some crystals jump during the phase transition. These thermosalient crystals are biomimetic, nonpolymeric self-actuators par excellance. Yet, due to exclusivity and individuality of the phenomenon, all present investigations have not yet resulted in the full elucidation of this mechanism, let alone enabled us to predict necessary prerequisites for its existence so we aimed our research towards new materials obtained by careful modification of compounds in which the effect was noticed. It is to be expected that this exotic and unexpected effect will be extremely sensitive to subtle changes in the chemical composition (change of the anion, change of the functional groups, change of the position of the functional group inside the aromatic ring) and these investigations will enable determination of necessary chemical/structural parameters so as to arise the thermosalient effect in the system. No matter if these compounds will be thermosalient in nature, or not, comparison between original systems and their derivatives will certainly provide new and helpful insight into elucidation of thermosalient behavior and offer new guidelines in the sense of targeted fabrication of new thermosalient materials. One of the first systematic studies of this fascinating effect, which paved a way towards its final elucidation, was conducted on the anticholinergic agent oxitropium bromide.2 It was shown that the unit-cell distortion is accompanied by a conformational change of the oxitropium cation, which triggers increased separation between the ion pairs in the lattice at nearly identical separation between the cation and the anion within each ion pair. 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 transition. In an attempt aimed at the targeted fabrication of thermosalient materials, a series of oxitropium salts was prepared. Oxitropium bromide was used as a starting reagent for preparation of several other oxitropium salts. Precursor compound was dissolved in minimal amount of water and the appropriate lead(II) salt was added under stirring conditions. After period of 1 h lead(II) bromide, which precipitated during the stirring, was removed by filtration. One of the prepared systems, oxitropium nitrate, showed thermosalient behaviour during observation on the hot-stage microscope and exhibited joyful jumps during heating (around 134ºC) and cooling (around 98ºC). Detailed structural (high temperature in-situ XRPD), thermal (DSC and TGA), spectroscopic (FT-IR), mechanistic and theoretical (DFT using state-of-the- art nonempirical vdW-DF-cx functional) study was conducted which revealed much information about the thermosalient effect in this system which are necessary to reach the final goal of understanding the nature and mechanism of this fascinating phenomenon.

thermosalient materials

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