Light Noble Gas Chemistry: Structures, Stabilities, and Bonding of Helium, Neon and Argon Compounds

Theoretically determined geometries are reported for the light noble gas ions Ng₂C²⁺, Ng₂N²⁺, Ng₂O²⁺, NgCCNg²⁺, NgCCH⁺, NgCN⁺, and NgNC⁺ (Ng = He, Ne, Ar) at the MP2/6-31G(d,p) level of theory. In a few cases, optimizations were carried out at CASSCF/6-31 G(d,p). The thermodynamic stability of the Ng compounds is investigated at MP4-(SDTQ)/6-311G(2df,2pd) for Ng = He, Ne and at MP4(SDTQ)/6-311G(d,p) for Ng = Ar. The structures and stabilities of the molecules are discussed in terms of donor–acceptor interactions between Ng and the respective fragment cation, by using molecular orbital arguments and utilizing the analysis of the electron density distribution and its associated Laplace field. Generally, there is an increase in Ng,X binding interactions of a noble gas molecule NgX with increasing atomic size of Ng. In some cases the Ne,X stabilization energies are slightly smaller than the corresponding He,X values because of repulsive p–π interactions in the neon compounds. The argon molecules are in all cases significantly stronger bound.