Non-covalent interactions in polymorphs of urea under pressure

The urea under high pressure is analyzed from a theoretical point of view using the quantum theory of atoms in molecules and the non-covalent interactions index. For this purpose, four urea crystal structures from X-ray, obtained for different pressures, were used to obtain their electronic structure with the relaxation of hydrogen atom positions using the density functional theory with periodic boundary conditions. From our analysis, the quantum chemistry predictors found N–H ⋯ O hydrogen bonds reported experimentally. Additionally, we found N–H ⋯ N hydrogen bonds, which have not been mentioned or underestimated by experimental arguments. We show that the N–H ⋯ N hydrogen bonds are essential in the description of the urea under high pressure. Furthermore, weak non-covalent interactions, not just hydrogen bonds, become crucial in stabilizing experimental structures under high pressure. This article presents a theoretical description of the molecular interactions involved in urea under high pressure for the first time.