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-H3+, recently proposed as intermediates in the sequestration of noble gases by H3+ 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.