Stabilization of HHeF by Complexation: Is it a Really Viable Strategy?

Abstract

Ab initio calculations at the MP2 and CCSD(T) levels of theory have disclosed the conceivable existence of fluorine-coordinated complexes of HHeF with alkali-metal ions and molecules M(+) (M(+)=Li(+)-Cs(+)), M(+)-OH(2), M(+)-NH(3) (M(+)=Li(+), Na(+)), and MX (M=Li, Na; X=F, Cl, Br). All these ligands L induce a shortening of the H-He distance and a lengthening of the He-F distance accompanied by consistent blue- and redshifts, respectively, of the H-He and He-F stretching modes. These structural effects are qualitatively similar to those predicted for other investigated complexes of the noble gas hydrides HNgY, but are quantitatively more pronounced. For example, the blueshifts of the H-He stretching mode are exceptionally large, ranging between around 750 and 1000 cm(-1). The interactions of HHeF with the ligands investigated herein also enhance the (HHe)(+)F(-) dipole character and produce large complexation energies of around 20-60 kcal mol(-1). Most of the HHeF-L complexes are indeed so stable that the three-body dissociation of HHeF into H+He+F, exothermic by around 25-30 kcal mol(-1), becomes endothermic. This effect is, however, accompanied by a strong decrease in the H-He-F bending barrier. The complexation energies, DeltaE, and the bending barriers, E*, are, in particular, related by the inverse relationship E*(kcal mol(-1))=6.9exp[-0.041DeltaE(kcal mol(-1))]. Therefore the HHeF-L complexes, which are definitely stable with respect to H+He+F+L (DeltaE approximately 25-30 kcal mol(-1)), are predicted to have bending barriers of only 0.5-2 kcal mol(-1). Overall, our calculations cast doubt on the conceivable stabilization of HHeF by complexation.

A metastable He–O bond inside a ferroelectric molecular cavity: (HeO)(LiF)2

“…Recent calculations predict that complexation with alkali metal cations and various small molecules also does not lead to sufficient stabilization of HHeF. 30 M(I)FHeO (M = Cs, NMe 4 ) described by Grochala are two more candidates for neutral He-containing molecules. 31 These are derivatives of the metastable À FHeO anion, first theorized by Hu et al in 2005.…”

Cationic Complexes of Hydrogen with Helium

“…However, so far no stable compound containing helium has been synthesized, except for structures where helium atoms are physically trapped inside fullerene cages 4. One promising type of novel compounds are noble-gas hydrides of the form HNgY where Ng is a noble-gas atom and Y an electronegative fragment 5, 6. A recent review2 lists 23 of these hydrides, but none including helium.…”

“…HHeF was initially predicted to be merely metastable 7. It might in fact be stabilized by the environment but has not yet been detected 6, 8…”

On the position of helium and neon in the Periodic Table of Elements

“…Subsequent studies using multireference methods have showed that the barrier is even smaller than initially thought, and the predicted lifetime of HHeF is only 157 fs (Takayanagi and Wada 2002). It was also suggested that HHeF cannot be sufficiently well stabilized by complexation (Giordani et al 2010). Some authors considered placing (or in situ generating of) HHeF molecules in the helium matrix and applying high pressure (Bihary et al 2002).…”