Cationic noble gas hydrides-2: A theoretical investigation on HNgHNgH+ (Ng = Ar, Kr, Xe)

The linear, centrosymmetric HNgHNgH+ (Ng = Ar, Kr, Xe) were characterized as true energy minima at the MP2, CCSD(T), and B3LYP levels of theory. The optimized geometries, atomic charges, AIM bond topologies, and harmonic frequencies point to ion–dipole complexes between a hydride anion and two covalent NgH+ cations, best formulated as (H–Ng+)2H−. The xenon cation HXeHXeH+ resulted thermochemically stable with respect to dissociation into XeHXe+ and 2H, and protected by a barrier of ca. 17–22 kcal mol−1 with respect to the largely exothermic extrusion of a Xe atom so to form XeH+ and H2. On the other hand, both HArHArH+ and HKrHKrH+, even though kinetically stable by ca. 5–15 and 12–19 kcal mol−1 with respect to dissociation into NgH+, Ng, and H2, are however largely unstable with respect to the loss of two H atoms and formation of NgHNg+. Therefore, they are predicted to be unstable even at the lowest temperatures. The HNgHNgH+ cations (Ng = Ar, Kr, Xe) are also considerably less stable than the isomeric clusters (Ng)2– H 3 + , recently proposed as intermediates in the sequestration of noble gases by H 3 + in protoplanetary objects. In any case, our calculations invite the theoretical investigation of the stability of these clusters with respect to dissociation into the low-energy fragments NgHNg+ and H2.