Theoretical predictions of the structure, gas-phase acidity, and aromaticity of tetrathiosquaric acid

Results of ab initio self-consistent-field and density functional theory ca lculations of the gas-phase structure, acidity (free energy of deprotonatio n, Delta G(0)), and aromaticity of tetrathiosquaric acid (3,4-dithiohydroxy -3-cyclobutene-1,2-dithione, H2C4S4) are reported. The global minimum found on the potential energy surface of tetrathiosquaric acid presents a planar conformation. The ZZ isomer was found to have the lowest energy among the three planar conformers and the ZZ and ZE isomers are very close in energy. The optimized geometric parameters exhibit a bond length equalization rela tive to reference compounds, cyclobutanedithione, and cyclobutenedithiol. T he computed aromatic stabilization energy by homodesmotic reaction is -18.4 (MP2(fu)/6-311+G**//RHF/6-311+G**) and -15.1 kcal/mol (B3LYP//6-311+G**//B 3LYP/6-311+G**). The aromaticity of tetrathiosquaric acid is indicated by t he calculated diamagnetic susceptibility exaltation (Lambda) -11.77(CSGT(IG AIM)-RHF/G-311+G**//RHF/6-311+G**) and -18.08 (CSGT(IGAIM)-B3LYP/6-311+G**/ /B3LYP/6-311+G**). Thus, tetrathiosquaric acid fulfils the geometric, energ etic and magnetic criteria of aromaticity. The most reliable theoretical ga s-phase acidities are Delta G(1(298K))0 = 303.7 and Delta G(2(298K))(0) = 3 94.1 kcaljmol. Hence, tetrathiosquaric acid is a stronger acid than squaric acid (3,4-dihydroxy-3-cyclobutene-1,2-dione, H2C4O4) Comparisons of the co mputed results of tetrathiosquaric acid with squaric acid have also been ma de. (C) 2000 John Wiley & Sons, Inc.