Theoretical prediction of the structures and properties of cyclic AlS2 and GaS2 systems at density functional theory and all-electron correlation levels

The geometries and bonding properties have been predicted for cyclic AlS2 and GaS2 species at DFT, MPn (n=2, 3, 4), QCISD(T) and CCSD(T) levels with the 6-311+G∗ basis set. Harmonic vibrational frequencies were determined using DFT and MP2 methods. The relevant energies were calibrated at high-order electron correlation levels. Each species possesses a 2A2 ground state with a higher-energy 2A1 state, lying 7.2 kcal/mol for AlS2 or 20.0 kcal/mol for GaS2 above at CCSD(T)/6-311+G∗ level, and the corresponding CCSD(T) dissociation energies are 64.0 (AlS2: 2A2) and 59.0 kcal/mol (GaS2: 2A2), respectively. Analysis indicates that the cyclic AlS2 and GaS2 should be classified as supersulfide, but they are not as ionic as NaO2 and less ionic than AlO2.