H₃⁺ as a trap for noble gases-3: Multiple trapping of neon, argon, and krypton in X_n H₃⁺ (n=1-3)

Abstract

Recent studies on the formation of XH3+ noble gas complexes have shown strategic implications for the composition of the atmospheres of the giant planets as well as for the composition of comets. One crucial factor in the astrophysical process is the relative abundances of the noble gases versus H3+. It is the context in which the possibility for clustering with more than one noble gas (XnH 3+ up to n=3) has been investigated for noble gases X ranging from neon to krypton. In order to assert our results, a variety of methods have been used including ab initio coupled cluster CCSD and CCSD(T), MP2, and density functional BH&HLYP levels of theory. All complexes with one, two, and three noble gases are found to be stable in the Ne, Ar, and Kr families. These stable structures are planar with the noble gases attached to the apices of the H3+ triangle. The binding energy of the nth atom, defined as the XnH3+ → X n-1H3+ +X reaction energy, increases slightly with n varying from 1 to 3 in the neon series, while it decreases in the argon series and shows a minimum for n=2 in the krypton series. The origin of this phenomenon is to be found in the variations in the respective vibrational energies. A topological analysis of the electron localization function shows the importance of the charge transfer from the noble gases toward H 3+ as a driving force in the bonding along the series. It is also consistent with the increase in the atomic polarizabilities from neon to krypton. Rotational constants and harmonic frequencies are reported in order to provide a body of data to be used for the detection in laboratory prior to space observations. This study strongly suggests that the noble gases could be sequestered even in an environment where the H3+ abundance is small. © 2009 American Institute of Physics.