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| - Mechanistic Aspects of the Solid-State Transformation of Ammonium Cyanate to Urea atHigh Pressure
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| - The chemical transformation of ammonium cyanate into urea has been of interest to many generations ofscientists since its discovery by Friedrich Wöhler in 1828. Although widely studied both experimentally andtheoretically, several mechanistic aspects of this reaction remain to be understood. In this paper, we applycomputational methods to investigate the behavior of ammonium cyanate in the solid state under high pressure,employing a theoretical approach based on the self-consistent-charges density-functional tight-binding method(SCC-DFTB). The ammonium cyanate crystal structure was relaxed under external pressure ranging from 0to 700 GPa, leading to the identification of five structural phases. Significantly, the phase at highest pressure(above 535 GPa) corresponds to the formation of urea molecules. At ca. 25 GPa, there is a phase transitionof ammonium cyanate (from tetragonal P4/nmm to monoclinic P21/m) involving a rearrangement of theammonium cyanate molecules. This transformation is critical for the subsequent transformation to urea. Thecrystalline phase of urea obtained above 535 GPa also has P21/m symmetry (Z = 2). This polymorph of ureahas never been reported previously. Comparisons to the known (tetragonal) polymorph of urea foundexperimentally at ambient pressure suggests that the new polymorph is more stable above ca. 8 GPa. Ourcomputational studies show that the transformation of ammonium cyanate into urea is strongly exothermic(enthalpy change −170 kJ mol-1 per formula unit between 530 and 535 GPa). The proposed mechanism forthis transformation involves the transfer of two hydrogen atoms of the ammonium cation toward nitrogenatoms of neighboring cyanate anions, and the remaining NH2 group creates a C−NH2 bond with the cyanateunit.
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