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

Abstract

Elemental helium is a prototypical noble gas and its atom sets the records for many physicochemical properties. With its two electrons in the closed 1s shell, He is the smallest, the least polarizable, the most difficult to ionize, the hardest (in Pearson's sense) and the most electronegative atom known. Helium gas is considered to be as close to an 'ideal gas' as possible and used as a standard of compressibility and pressure. As a consequence of its closed-shell electronic configuration, helium also exhibits chemical inertness: non-charged species which contain chemically bound helium have not been synthesized to date. Here we are able to predict, using quantum mechanical methods, a small neutral molecule, (HeO)(LiF)(2), which contains a helium atom chemically bound to oxygen. The molecule is planar with the polarized He(δ+)O(δ-) unit embedded anti-parallel in-between two co-aligned LiF dipoles. We calculate its selected physicochemical properties as well as suggest possible decomposition and synthesis pathways. Successful preparation of neutral species, (HeO)(LiF)(2) or related molecules, would break down the paradigm of helium's inertness.

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

“…The (HeO)(LiF) 2 molecule is also different from other neutral species bearing helium by the fact that the noble gas atom has only one close neighbor and the trans position is free (this helps to avoid hypervalence at helium). Remarkably, the neon analogue does not correspond to a minimum (Grochala 2012) but the heavier noble gases form the minima on the singlet potential energy surface (Szarek and Grochala 2015). Moreover, there is a striking similarity between the molecular geometries of (HeO)(LiF) 2 and (BeO)(LiF) 2 (Fig.…”

“…The last type of molecules containing covalently bound He, which have been theorized in the past, are the derivatives of helium monoxide, HeO, with small molecules which exhibit substantial dipole moment (Grochala 2009a(Grochala , 2012. These include: (HeO)(MF) where M = Cs, NMe 4 , as well as (HeO)(LiF) 2 .…”

“…On the other hand, metastable (HeO)(LiF) 2 is best described as HeO molecule embedded into a ferroelectric cavity composed of two parallel LiF dipoles (Grochala 2012). This interesting minimum on the potential energy surface features a short covalent He-O bond at * 1.15 Å ; the bond is slightly polarized with the electrons transferred from helium to oxygen (He…”

Bonding Motifs of Noble-Gas Compounds As Described by the Local Electron Energy Density

“…52,74,75 It is also of interest to note here the theoretical prediction of related neutral compounds such as (LiF) 2 (HeO), and other noble gas monoxides. 76,77 In general, the interaction of Ng atoms with anionic species produces weakly-bound non covalent complexes, especially for the lightest He, Ne, and…”

On the directional character of orbital compression: A model study of the electric properties of LiH–(He) complexes

“…It is also worth mentioning, that the polarizability of He atom is about 30% smaller than that of the hydrogen atom. In fact, helium is the least polarizable as well as the smallest (considering its atomic radius), the most difficult to ionize, the hardest (in Pearson's scale) and the most electronegative atom known [38]. This makes He atoms particularly suitable for model studies concerning the pure spatial confinement effect, what has been shown in several theoretical studies in the past [14,19,22,[39][40][41][42][43][44].…”