Vicinal dihalonium ions: diprotonated and dimethylated chlorine [H2Cl2(2+), (CH3)2Cl2(2+)] and bromine [H2Br2(2+), (CH3)2Br2(2+)] dications.

High level ab initio calculations at the MP2/cc-pVTZ, CCSD(T)/cc-pVTZ, and CASSCF(6,6)/cc-pVTZ levels were performed to investigate geometries and energies of superelectrophilic diprotonated, and dimethylated molecular chlorine (Cl2) and bromine (Br2) dications. Diprotonated chlorine and diprotonated bromine dications 3a and 6a, respectively, were found to be lowest energy minima. The isomeric dications, 3b and 6b, are also minima on the potential energy surfaces but they are significantly less stable than the structures 3a and 6a by 33.6 and 30.9 kcal/mol, respectively. On the basis of computed G2 energies, proton affinities and related thermodynamic parameters were also calculated. Dications 3a and 6a have substantial kinetic barriers for deprotonation. Their homolytic dissociation are however facile. Dimethylated molecular chlorine and bromine dications 3g and 6g, respectively, were also found to be global energy minima. These vicinal dihalonium or the corresponding protosolvated species are expected to form either in the superacidic media or in the gas phase.