Complexes of Alkali Metal Cations and Molecular Hydrogen: Potential Energy Surfaces and Bound States

Abstract

Complexes between metal cations and molecular hydrogen are systems quite amenable for precise spectroscopic and theoretical studies and, at the same time, they are relevant for applications in hydrogen storage and astrochemistry. In this work we report new intermolecular potential energy surfaces and rovibrational states calculations for complexes involving molecular hydrogen and alkaline metal cations, M +-H 2 (M + =Na + , K + , Rb + , Cs +). The intermolecular potentials, formulated in an internally consistent way to emphasize differences in the properties of the systems, are represented by simple analytical expressions whose parameters have been optimized from comparison with accurate ab initio calculations. Properties of the low-lying bound states-binding energies, frequencies, rotational constants-are compared with previous measurements or computations and an overall good agreement is achieved, supporting the reliability of the present formulation. Variations of these properties as a function of the cation size and isotopic substitution, with a proper sequence of ortho and para rotational levels, are also discussed.

Attachment of Hydrogen Molecules to Atomic Ions (Na+, Cl−): Examination of an Adiabatic Separation of the H2 Rotational Motion

“…and compare them with the energies resulting from a fully coupled calculation. The result is quite satisfactory, as we obtain E 0 p = À 111.98 meV and E 0 o = À 103.01 meV, in very good agreement with the results of close-coupling computations, [24] À 111.86 and À 102.94 meV, respectively. In Figure 1 (upper panel), it can be noticed that the V ad p R ð Þ potential is more attractive than the spherically averaged one (Av-PsAt).…”

“…These parameters have been optimized from comparisons with high level ab initio calculations, which, in the case of H 2 -Na + , have been reported in Ref. [24]. Details on new ab initio calculations and optimized parameters for the H 2 -Cl À interaction are reported in the SI.…”

“…One aspect of interest for hydrogen storage applications is the determination of the ion coordination number, i. e., the number of H 2 molecules that are directly attached to the ion, forming the first shell of the cluster. Since H 2 -ion binding energies are generally larger for oH 2 than for pH 2 , [24,25] it can be expected that more ortho molecules will be attached to the ion.…”

Helium nanodroplets as an efficient tool to investigate hydrogen attachment to alkali cations

“…An important point to be emphasized is the different representation of the long range in Na + (H 2 ) n and Na 2 + (H 2 ) n clusters stability which is controlled by the induction attraction. In particular, while Na + is a charge point, 36–38 Na 2 + is a strongly polarizable and elongated ionic diatom 39,40 showing an equilibrium distance, R e , of 3.71 Å and a strongly anisotropic charge distribution, as suggested by its high electric quadrupole moment (see the ESI†). Therefore, the radial dependence of the induction attraction for configurations involving Na 2 + aligned along the intermolecular distance (see configurations L and T b in the Potential Energy Surfaces section of the ESI†) assumes the canonical R −4 dependence (such as that for Na + ) only asymptotically ( R ⪢ R e ), while in the region of interest for cluster stability ( R ∼ R e ) the R −6 dependence, typical of a dipole, is found to be more appropriate (see Fig.…”

“…An important point to be emphasized is the different representation of the long range in Na + (H 2 ) n and Na + 2 (H 2 ) n clusters stability which is controlled by the induction attraction. In particular, while Na + is a charge point [36][37][38] , Na + 2 is a strongly polarizable and elongated ionic diatom 39,40 showing an equilibrium distance, R e , of 3.71 Å and a strongly anisotropic charge distribution, as suggested by its high electric quadrupole moment (see SI). Therefore, the radial dependence of the induction attraction for configurations involving Na The expressions of the different terms in Eq.…”