Helium insertion reactions with ammonia and water under pressure

The noble gas elements are usually quite inert to chemical reactions due to their closed shell configurations. He is the most stable noble gas since its ionization energy is almost twice as large as that of Xe. It was recently demonstrated that He could react with a large number of ionic compounds with unequal number of cations and anions due to the substantial change of the electrostatic energy under high pressure. In this work, we explore and show the reactivity of He with two molecular crystals, ammonia (NH₃) and ice (H₂O) by density functional theory calculations. We found that NH₃ and He can form stable structure above 50 GPa with a NH₃:He ratio of 1:1, while H₂O and He can form stable structure above 300 GPa with a H₂O:He ratio of 2:1. Although change of the electrostatic interaction is the driving force for the He insertion under high pressure, the mechanism is very different between ammonia and ice. This work extends the reactivity of He into a new area of molecular crystals, showing the richness of the chemistry of this most inert element in the periodic table. Since He, NH₃ and H₂O are the major components of giant gas planets, the new chemistry revealed in our work is important for the understanding of the structure and the evolution of these planets.