Neon clathrate
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. 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 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.
NeAuF and NeBeS have been isolated in noble gas matrixes. NeBeCO3 has been detected by infrared spectroscopy in a solid neon matrix. It was made from beryllium gas, dioxygen and carbon monoxide.
Van der Waals molecules with metals include LiNe.
More Van der Waals molecules include CF4Ne and CCl4Ne, Ne2Cl2, Ne3Cl2, I2Ne, I2Ne2, I2Ne3, I2Ne4, I2NexHey (x=1-5, y=1-4).
Van der Waals molecules formed with organic molecules in gas include aniline,dimethyl ether,1,1-difluoroethylene,pyrimidine,chlorobenzene,cyclopentanone,cyanocyclobutane, and cyclopentadienyl.
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