Prediction of crystalline structure and properties of nitryl cyanide solids at high pressure.

Nitryl cyanide is a compound containing a strong oxidizer (NO₂ group) adjacent to an excellent fuel source (CN group) and is a promising high-energy-density material. The structure of the nitryl cyanide crystal at high pressure is predicted using density functional theory (DFT) and evolutionary algorithm. Calculated heat of formation of the trimer (NCNO₂)*3 was determined using GAUSSIAN and interpolation of heats of formation of CHNO compounds as 67.622 kcal/mol for gas phase and 48.1 kcal/mol for solid-state. The mass density of the solid was estimated using the total molecular volume method and found to be 1.87 g/cc. The structure of the nitryl cyanide crystal was found using the molecular option of evolutionary code USPEX by packing up to six NCNO₂ molecules. The lowest enthalpy symmetry group of the crystal was found to be CC (CS-4), with a density of 3.1 g/cc at 20 GPa. Phonon dispersion calculations indicate dynamical stability of the new structure similar to the planes of NCNO₂ trimer connected with covalent bonds between the planes. The intensity of Raman and IR spectra of the nitryl cyanide crystal are calculated and main peaks are interpreted in terms of atomic and collective vibrations.