Neon compounds

Neon compounds were long believed not to exist. Neutral neon-containing molecules were only discovered in the twenty-first century, and even today are not well known. Neon has a high first ionisation potential of 21.564 eV, which is only exceeded by that of helium (24.587 eV). This means that ionic compounds use too much energy to make. Neon's polarisability of 0.395 Å³ is the second lowest of any element (only helium's is more extreme). Low polarisability means there will be little tendency to stick to other atoms.^[1] However, there are molecular ions which contain neon, as well as temporary excited neon-containing molecules called excimers. Several neutral neon molecules have been predicted to be stable, but have not yet been found to exist. Neon has been shown to crystallise with other substances to form clathrates or Van der Waals solids.

Van der Waals molecules

Van der Waals molecules are those where neon is held onto other components by London dispersion forces. The forces are very weak, so the bonds will be disrupted if there is too much molecular vibration, which happens if the temperature is too high, when the temperature is above that of solid neon.

Neon atoms themselves can be linked together to make clusters of atoms. The dimer Ne₂, trimer Ne₃ and neon tetramer Ne₄ have all been characterised by Coulomb explosion imaging. The molecules are made by an expanding supersonic jet of neon gas. The neon dimer has an average distance of 3.3 Å between atoms. The neon trimer is shaped approximately like an equilateral triangle with sides 3.3 Å long. However the shape is floppy and isosceles triangle shapes are also common. The first excited state of the neon trimer is 2 meV above the ground state. The neon tetramer takes the form of a tetrahedron with sides around 3.2 Å.^[2]

NeAuF^[3] and NeBeS^[4] have been isolated in noble gas matrixes.^[5] NeBeCO₃ has been detected by infrared spectroscopy in a solid neon matrix. It was made from beryllium gas, dioxygen and carbon monoxide.^[6]

Van der Waals molecules with metals include LiNe.^[7]

More Van der Waals molecules include CF₄Ne and CCl₄Ne, Ne₂Cl₂ and Ne₃Cl₂.^[8]

Van der Waals molecules formed with organic molecules in gas include aniline,^[9] dimethyl ether,^[10] 1,1-difluoroethylene,^[11] pyrimidine,^[12] chlorobenzene,^[13] cyclopentanone,^[14] cyanocyclobutane,^[15] and cyclopentadienyl.^[16]

Ligands

Neon can form a very weak bond to a transition metal atom as a ligand, for example Cr(CO)₅Ne,^[17] Mo(CO)₅Ne, and W(CO)₅Ne.^[6]

NeNiCO is predicted to have a binding energy of 2.16 kcal/mol. The presence of neon changes the bending frequency of Ni−C−O by 36 cm⁻¹.^[18][19]

Solids

High pressure Van der Waals solids include (N₂)₆Ne₇.^[20]

Neon hydrate or neon clathrate, a clathrate, can form in ice II at 480 MPa pressure between 70 K and 260 K.^[21] Other neon hydrates are also predicted resembling hydrogen clathrate, and those of helium. These include the C₀, ice I_h and ice I_c forms.^[21]

Neon atoms can be trapped inside fullerenes such as C₆₀ and C₇₀. The isotope ²²Ne is strongly enriched in carbonaceous chondrite meteorites, by more than 1,000 times its occurrence on Earth. This neon is given off when a meteorite is heated.^[22] An explanation for this is that originally when carbon was condensing from the aftermath of a supernova explosion, cages of carbon form that preferentially trap sodium atoms, including ²²Na. Forming fullerenes trap sodium orders of magnitude more often than neon, so Na@C₆₀ is formed. rather than the more common ²⁰Ne@C₆₀. The ²²Na@C₆₀ then decays radioactively to ²²Ne@C₆₀, without any other neon isotopes.^[23] To make buckyballs with neon inside, buckminsterfullerene can be heated to 600 °C with neon under pressure. With three atmospheres for one hour, about 1 in 8,500,000 molecules end up with Ne@C₆₀. The concentration inside the buckyballs is about the same as in the surrounding gas. This neon comes back out when heated to 900 °C.^[24]

Neon can be trapped inside some metal-organic framework compounds. In NiMOF-74 neon can be absorbed at 100 K at pressures up to 100 bars, and shows hysteresis, being retained till lower pressures. The pores easily take up six atoms per unit cell, as a hexagonal arrangement in the pores, with each neon atom close to a nickel atom. A seventh neon atom can be forced under pressure at the centre of the neon hexagons.^[25]

Ions

Ionic molecules can include neon, such as the clusters Ne
mHe+
n where m goes from 1 to 7 and n from 1 to over 20.^[26] HeNe⁺ (helium neide) has a relatively strong covalent bond. The charge is distributed across both atoms.^[27]

When metals are evaporated into a thin gas of hydrogen and neon in a strong electric field, ions are formed that are called neides. Ions observed include TiNe⁺, TiH₂Ne⁺, ZnNe²⁺ , ZrNe²⁺, NbNe²⁺, NbHNe²⁺, MoNe²⁺, RhNe²⁺, PdNe⁺, TaNe³⁺, WNe²⁺, WNe³⁺, ReNe³⁺, IrNe²⁺, AuNe⁺ (possible).^[28]

SiF₂Ne²⁺ can be made from neon and SiF2+
3 using mass spectrometer technology. SiF₂Ne²⁺ has a bond from neon to silicon. SiF2+
3 has a very weak bond to fluorine and a high electron affinity.^[29]

NeCCH⁺, a substituted acetylene, is predicted to be energetically stable by 5.9 kcal/mol, one of the most stable organic ions.^[30]

Neonium

The ion NeH⁺ formed by protonating neon, is called neonium. It is produced in an AC electric discharge through a mixture of neon and hydrogen with more produced when neon outnumbers hydrogen molecules by 36:1.^[31] The dipole moment is 3.004 D.^[31]

Neonium is also formed by excited dihydrogen cation reacting with neon: Ne + H₂^+* → NeH⁺ + H^[32]

The infrared spectrum around 3μm has also been measured.^[33]

Excimers

The Ne*
2 molecule exists in an excited state in an excimer lamp using a microhollow cathode. This emits strongly in the vacuum ultraviolet between 75 and 90 nm with a peak at 83 nm. There is a problem in that there is no window material suitable to transmit these short wavelengths, so it must be used in a vacuum. If about one part in a thousand of hydrogen gas is included, most of the Ne*
2 energy is transferred to hydrogen atoms and there is a strong monochromatic Lyman alpha emission at 121.567 nm.^[34]

Cesium can form excimer molecules with neon CsNe^*.^[35]

A hydrogen-neon excimer is known to exist. Fluorescence was observed by Möller due to bound free transition in a Rydberg molecule of NeH^*. NeH is metastable and its existence was proved by mass spectroscopy in which the NeH⁺ ion is neutralized and then reionized.^[36] The spectrum of NeH includes lines at 1.81, 1.60 and I .46 eV, with a small band at 1.57 eV^[37] The bondlength in NeH is calculated as 1.003 Å.^[36]

A helium neon excimer can be found in a mixed plasma or helium and neon.^[38]

Some other excimers can be found in solid neon, including Ne+
2O−
which has a luminescence peaking around 11.65 eV, or Ne+
2F−
luminescing around 10.16–10.37 eV and 8.55 eV.^[39]

Minerals

Bokiy's crystallochemical classification of minerals included "compounds of neon" as type 82. However, no such minerals were known.^[40]

See also

• Noble gas compound

References