Theoretical studies of the gas-phase acidity and aromaticity of 3,4-dithiosquaric acid

The conformation, aromaticity and gas-phase acidity (free energy of deprotonation, Delta G degrees) of 3,4-dithiosquaric acid (3,4-dimercapto-3-cyclobutene-1,2-dione) were calculated at the SCF and MP2 and B3LYP levels using 6-311G (d, p) and 6-311 + G (d, p) basis sets. The global minimum found on the potential energy surface of 3,4-dithiosquaric acid presents a planar conformation. The ZZ isomer was found to be the most stable of the three planar conformers. The aromatic stabilization energy(ASE) and magnetic susceptibility exaltation (Lambda) were also computed by using the homodesmotic reaction and RHF-CSGT (Continues Set of Gauge Transformations) and RHF-IGAIM (a slight variation on CSGT) and B3LYP-CSGT (IGAIM) methods at the 6-311 + G(d, p) level. The calculated ASE and Lambda were negative values, indicating that 3,4-dithiosquaric acid is aromatic. Thus the titled compound fulfilled the geometrical, energetic and magnetic criteria of aromaticity. The most reliable theoretical gas-phase acidity are Delta G(1, 298 K)degrees = 1 295.4 kJ/mol and Delta G(2, 298 K)degrees = 1 691, 2 kJ/mol.