Why are Nanothreads 1D?

In the remarkable nanothread formation reaction, the threads consistently align to an applied uniaxial stress imposed by the opposed anvil device (diamond cell or double toroidal anvils in a Paris-Edinburgh Press) to form hexagonal crystals, despite their formation from a 3-dimensional, monoclinic benzene molecular crystal. Our hypothesis is that a subset of benzene stacks parallel to the uniaxial stress in either single crystal or polycrystalline benzene are selected for reaction, as both form single-crystal nanothreads (see JACS 139, 16343 (2017).). Subsequent collapse into 1D nanothreads is favored because the volume of activation is reduced most (and reaction rate increased most) by thread-forming reactions that maximize initial contact areas between a small aromatic molecule and a growing thread, i.e., a “pancake stack” geometry  (and slow, room-temperature growth disfavors alternative reaction pathways). Hoffmann has shown that [4+2] cycloadditon reactions, with negative volumes of reaction, are likely, and preliminary solid-state NMR results on bonding within threads has intriguing suggestions along similar lines. The barriers against reaction perpendicular to the thread axis are higher (and thus kinetics slower) for molecules as small as benzene.

Uniaxial stress on an object
Uniaxial stress that is slightly larger in one direction (arrows) will bring reactant molecules closer together in that direction first.

Interrogating this hypothesis with integrated theory and experiment is a key Phase I activity. We will use single-crystal X-ray diffraction to reveal the crystallographic relationship between the growing nanothreads and their parent molecular crystals to understand the solid-state geometry of reaction, while also monitoring the in-situ evolution of single-crystal diffraction spots and using SAXS to study the assembly and packing of growing threads