Structural Basis for Jumping of Organic Crystals

Sinha, Shashi Bhushan (2012) Structural Basis for Jumping of Organic Crystals. Masters thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

Materialization of compounds which are capable of reflecting the changes in their molecular geometry by some mechanical motion has long been a challenging task. Such systems are interesting from research perspective- understanding the mechanism precisely and be able to correlate the molecular motions by some topological parameter (such as changes in mechanical property). Our study is based on establishing a link between the mechanical properties and the changes in molecular geometry during jumping for a group of systems, exhibiting a strong thermosalient effect (“jumping crystal” effect). The proposal of a mechanism for the jumping behavior is still in its formation and existence of such a link may expedite the process. 1,2,4,5- Tetrabromobenzene [TBB] and (Phenylazophenyl) palladium Hexafluoroacetylacetonate [PHA] possesses rich crystal chemistry, most remarkably exhibiting a strong thermosalient effect (“jumping crystal” effect). The differences in mechanical properties of the two phases upon jumping were investigated by a combination of structural, microscopic and quantitative techniques, providing data on which to find a link between the two phenomena. Our qualitative and quantitative analysis suggests that there is indeed a link between the mechanical properties of the two phases and their molecular motions upon jumping for both the systems (TBB and PHA). The reversible phase change in TBB is a result of a miniscule change w.r.t. the molecular packing of the two polymorphs. The molecules being rigid showed negligible conformational change. The mechanical property change between the two polymorphs of TBB is also very less- both the polymorphs being brittle. The nanoindentation revealed the deformation to be elastic in nature thus establishing the brittleness quantitatively. The PHA crystals displayed a variety a physical and mechanical property change in the two polymorphs. The mechanical property of the low temperature polymorph was highly anisotropic and a structural basis has been proposed. The substantial differences in the mechanical property of the two polymorphs were studied qualitatively too. The quantitative analysis is to be done. There are strikingly large differences in the mechanical property for two polymorphs of PHA. Concomitantly large and irreversible changes in the molecular arrangement (conformational change accompanied with molecular motions) during the phase transformation upon jumping is a demonstration of the fact that there can indeed be a link between the mechanical property and the jumping phenomena.

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