The Helical Structure of Diallylamine in the Solid State
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ORIGINAL PAPER
The Helical Structure of Diallylamine in the Solid State Chloe J. Pugh1 · Craig M. Robertson1 · Alexander Steiner1 Received: 7 June 2019 / Accepted: 16 November 2019 © The Author(s) 2019
Abstract In the solid state diallylamine forms supramolecular helices with four molecules per pitch that are held together by hydrogen bonding. The helical structure is the result of competing length scales at which hydrogen bonding and second-neighbour Van-der-Waals interactions occur. The structure features two crystallographically independent helices and four unique molecules in the asymmetric unit (Z′ = 4). The high Z′ value is partly a consequence of the centrosymmetric pseudo-hexagonal packing of helical columns, which is incompatible with helical spacegroup symmetries. Graphic Abstract
Keywords Low-melting solids · X-ray crystal structure · Supramolecular helix · Hydrogen bonding · High Z′
Introduction Helical arrangements are frequently observed in crystal structures of small molecules that form hydrogen bonded chains such as simple alcohols [1–3] or amines [4, 5]. Recently, we have found that many of these helices are only Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10870-019-00816-2) contains supplementary material, which is available to authorized users. * Alexander Steiner [email protected] 1
Department of Chemistry, University of Liverpool, Crown Street, Liverpool L69 7ZD, UK
stable when being reinforced by the crystal packing but would otherwise collapse into ring structures. In contrast, helices of dialkylamines equipped with small alkyl groups have shown to be inherently stable [6]. Out of this series diethylamine is the smallest and simplest molecule that has a supramolecular helix as its most stable aggregate. The two ethyl groups flank the central H-bonding site which is sufficient to prevent ring formation. The helical twist is a consequence of competing length scales of hydrogen bonding and second neighbour interactions between alkyl groups. Larger substituents, on the other hand, inhibit chain formation while methyl groups are too small to induce a helical twist. We were interested how diallylamine would fit into this series
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since it is of similar size and shape as dialkylamines that support supramolecular helices.
Experimental Diallylamine was purchased from Sigma-Aldrich and used without further purification. It is a liquid at room temperature; its melting point was measured at 185 K (− 88 °C). Single crystals were obtained using the zone-melting technique described by Boese and Nussbaumer [7]: The liquid sample was filled into a capillary of 0.5 mm diameter, which was mounted onto the diffractometer. The sample was flashfrozen at 100 K using a Cryostream (Oxford Cryosystems) and then subjected to successive heating scans along the capillary using an infrared laser until sufficiently large single crystals had formed. Intensity data were collected on a Bruker Apex II diffractometer with Mo K α
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