Chemical Aristocracy: He3 Dication and Analogous Noble-Gas-Exclusive Covalent Compounds

Herein, we predict the first set of covalently bonded triatomic molecular compounds composed exclusively of noble gases. Using a combination of double-hybrid DFT, CCSD(T), and MRCI+Q calculations and a range of bonding analyses, we explored a set of 270 doubly charged triatomics, which included various combinations of noble gases and main group elements. This extensive exploration uncovered nine noble-gas-exclusive covalent compounds incorporating helium, neon, argon, or combinations thereof, exemplified by cases such as He32+ and related systems. This work brings to light a previously uncharted domain of noble gas chemistry, demonstrating the potential of noble gases in forming covalent molecular clusters.

where  ⃗  represents the spatial and spin coordinates of the i-th electron.The superscripts (1), (2), …, indicate the antisymmetrized wavefunctions of the electron groups with  (1) ,  (2) , … ,  (𝑁) electrons.Moreover, the wavefunctions and orbitals of each group are orthogonal.The antisymmetrizing operator  only contains permutations involving one electron from each group.The VB2000 software is based on the use of GPF wavefunctions, allowing users to select a specific description for each electron group, such as HF, classical VB, SCGVB, or CASVB.Specifically, in SCGVB-PP calculations, the core electrons are treated as an HF group, while the valence electrons are distributed into groups consisting of two electrons, described using the SCGVB approach.
The GPF-EP 51 method employs a GPF to express first and second-order reduced density matrices as interference and reference (quasi-classical) densities.For a single group, the expressions for the quasiclassical and interference densities are as follows: where  indicates the group under consideration,   denotes the number of electrons in this group, 〈, 〉  represents the interference density associated with the orbitals   and   , and (|) corresponds to the elements of the density matrix in the orbital basis set.Specifically: with (, ) being the overlap integral between the orbitals   and   .
This approach can be extended to the pair density, resulting in a partition of the total energy of the system as follows: [] represents the total reference energy, E[I] and E[II] denote the first and second-order interference energies, while [] accounts for the total intergroup exchange interaction, arising from the antisymmetry of the GPF.In other words, [] introduces a symmetry correction to the reference energy due to the separation of the N-electron wavefunction into groups.The sum [] + [] corresponds to a (symmetrycorrected) quasi-classical contribution, and the sum [] + [] corresponds to the interference contribution.This framework has found successful applications in describing chemical bond formation and the role of quantum interference in bonding, as evidenced by a substantial body of literature focused on various case studies.S13,S19-S34 Additional Results Table S1.Calculated bond lengths (Å) using various methods with the aug-cc-pVTZ basis set.*AQCC/aug-cc-pVQZ Table S3.Mulliken and ChelpG atomic charges obtained using various methods with the aug-cc-pVTZ basis set.

Fig. S1 :Fig. S2 :Fig. S3 :
Fig.S1: SCGVB-PP orbitals at the SCGVB-PP/aug-cc-pVTZ level of theory.For Ar3 2+ , four valence electrons were treated in one GVB group and the remaining electrons are placed in a Hartree-Fock group.The numbers in black represent the bond lengths at the CCSD(T)/aug-cc-pVTZ level, while the numbers in red denote the ChelpG atomic charges.

Table S4 .
Dissociation paths calculated at the CCSD(T)/aug-cc-pVTZ level with and without zero-point energy (ZPE).For He + , the full configuration interaction (FCI) method was employed with the same basis set.