Understanding the Balance of Entropy and Enthalpy in Hydrogen-Halide Noncovalent Bonding.

Hydrogen bonds are of great scientific interest, determining the free energy landscape and hence chemical and physical properties of many materials systems, for example, the hybrid organic-inorganic perovskites. Although these interactions are critical, understanding them is difficult in complex, multicomponent systems; hydrogen halides are ideal as simple binary model compounds for understanding the role of hydrogen bonding in physical properties like phase transitions. Here we investigate the orthorhombic low-temperature phase and the cubic high-temperature phase in HX (X = F, Cl, Br, or I) systems to understand how different hydrogen-halide bonds influence free energy profiles. We show that hydrogen fluoride has a qualitatively different behavior due to strong hydrogen bonding and hence a very different vibrational entropy. Heavier halides are in contrast rather similar in their physical properties; however, dispersion interactions play a more crucial role in these. These results have implications for the rational design of materials with hydrogen-halide bonds and tuning material properties in systems like mixed anion CHNHPbX perovskites.