C-H bond activated by high pressure in Acetonitrile - Drs. Haiyan Zheng and Kuo Li
AUGUST 29, 2016
Acetonitrile (CH3CN) is the simplest and one of the most stable nitriles. It is well known that the reaction usually occur on the CºN triple bond rather than the inert C-H bond. New work from a team of HPSTAR scientists co-led by Drs. Haiyan Zheng and Kuo Li found that the C-H bond can be activated by the cyano group under high pressure. The hydrogen atom transfers from the CH3 to CN along the CH...N hydrogen bond which produces an amino group and initiates the polymerization. This stories is published in Angew. Chem Int. Ed. (doi: 10.1002/anie.201606198).
Based on the general rule of organic chemistry reaction at ambient condition, the CºN triple bond usually undergoes addition reaction. The same reaction was also discovered in the pressure-induced polymerization of the nitriles including the cyanogens and cyanoacetylene. Thus, the CN group was also supposed to be the main reaction site in acetonitrile before this new study.
"The chemical reaction of molecules under solid state can be completely different with what is should be according to the common sense", said Haiyan. "The reactivity of the molecules in the gas phase or in solution is known, but we still lacked of the knowledge of the solid state reaction." she added.
By multiple in-situ techniques including Raman, IR, neutron diffraction, the neutron pair distribution function, solid state NMR as well as the metadynamic simulation, the team discovered that the acetonitrile undergoes a completely new reaction route. The topology of the crystal structure governed by the weak CH...N hydrogen bonds play a critical role in controlling the reactivity. The hydrogen atom transfers from the CH3 to CN along the hydrogen bond, which produces an amino group and initiates polymerization to form a dimer, 1D chain and 2D nanoribbon with mixed sp2 and sp3 bonded carbon. Finally, it transforms into a graphitic polymer by eliminating ammonia.
"Our studies show that the new chemistry reaction rule can be discovered under high pressure and explicitly demonstrate the solid state reaction under high pressure is directed by the crystal structure", Kuo said, "Even the weak CH...N hydrogen bonds can induce the hydrogen migration under high pressure and this reaction type may represent a prototype reaction happened in the crystal structure containing the heteronuclear hydrogen bonds" he added.
Caption: The crystal structure of acetonitrile under high pressure obtained by the in-situ neutron diffraction and the schematic diagram of the reaction of acetonitrile under high pressure at room temperature.