Understanding and modulating the high-energy properties of noble-gas hydrides from their long-bonding: an NBO/NRT investigation on HNgCO+/CS+/OSi+ and HNgCN/NC (Ng = He, Ar, Kr, Xe, Rn) molecules

Abstract

The noble-gas hydrides, HNgX (X is an electronegative atom or fragment), represent potential high-energy materials because their two-body decomposition process, HNgX → Ng + HX, is strongly exoergic. Our previous studies have shown that each member of the HNgX (X = halogen atom or CN/NC fragment) molecules is composed of three leading resonance structures: two ω-bonding structures (H–Ng⁺ :X⁻ and H:⁻ Ng⁺–X) and one long-bonding structure (H^∧X). The last one paints a novel -type long-bonding picture. The present study focuses on the relationship between this novel bonding motif and the unusual energetic properties. We chose HNgCO⁺/CS⁺/OSi⁺/CN/NC, with the formula HNgAB (Ng = He, Ar, Kr, Xe, Rn; AB = CO⁺/CS⁺/OSi⁺/CN/NC) as the research system. We first investigated the bonding of HNgCO⁺ and its analogous HNgCS⁺/OSi⁺ species using NBO/NRT methods, and quantitatively compared the bonding with that in HNgCN/NC molecules. NBO/NRT results showed that each of the HNgCO⁺/CS⁺/OSi⁺ molecules could be better represented as a resonance hybrid of ω-bonding and long-bonding structures, but the long-bonding is much weaker than that in HNgCN/NC molecules. Furthermore, we introduced the long-bonding concept into the rationalization of the high-energy properties, and found a good correlation between the highly exothermic two-body dissociation channel and the long-bond order, b_H–A. We also found that the long-bond order is highly tunable for these noble-gas hydrides due to its dependence on the nature of the electronegative AB fragments or the central noble-gas atoms, Ng. On the basis of these results, we could optimize the energetic properties by changing the long-bonding motif of our studied molecules. Overall, this study shows that the long-bonding model provides an easy way to rationalize and modulate the unusual energy properties of noble-gas hydrides, and that it is helpful to predict some noble-gas hydrides as potential energetic materials.