Hydrogen bonds and elastic anisotropy of nitrile molecular crystals: an investigation from first-principles

Atomistic calculations based on semi-classical dispersion-corrected density functional theory were performed to investigate bonding and elastic properties of low temperature polymorphs of three nitrile molecular crystals. For that purpose, the DFT-PBE-D3ijk method, which account for both pair and three-body interactions, was applied. The analysis of Hirshfeld surfaces demonstrated that the (C-N…H) hydrogen bonds are the strongest and the dominant ones in the three nitrile molecular crystals. The latter were found to share a common structural feature, consisting of an antiparallel orientation of the terminal nitrile groups at their constituent molecules. Our prediction of the elastic properties demonstrated that malonitrile has the highest Young modulus and succinonitrile has the lower one. The three nitrile crystals are expected to have similar resistance to volume variation while succinonitrle is predicted to exhibit the lowest resistance to shape change. Otherwise, succinonitrile was found to be ductile while malononitrile and acetonitrile were shown to be brittle. Elasticity anisotropy analysis demonstrated that the directions of maximum Young's modulus are always nearly parallel to the direction of alignment of strong (C-H…N) hydrogen bonds in the three nitrile crystals. The present analyses are relevant for understanding the structure and the properties of more complex high temperature phases.