Theoretical predictions of the structure, gas-phase acidity and aromaticity of 1,2-diseleno-3,4-dithiosquaric acid

Results of ab initio self-consistent-held (SCF) and density functional theo ry (DFT) calculations of the gas-phase structure, acidity (free energy of d eprotonation, DeltaG(o), and aromaticity of 1,2-diseleno-3,4-dithiosquaric acid (3,4-dithiohydroxy-3-cyclobutene-1,2-diselenone, H2C4Se2S2) are report ed. The global minimum found on the potential energy surface of 1,2-diselen o-3,4-dithiosquaric acid presents a planar conformation. The ZZ isomer was found to have the lowest energy among the three planar conformers and the Z Z and ZE isomers are very close in energy. The optimized geometric paramete rs exhibit a bond length equalization relative to reference compounds, cycl obutanediselenone, and cyclobutenedithiol. The computed aromatic stabilizat ion energy (ASE) by homodesmotic reaction (Eq 1) is -20.1 kcal/mol (MP2(fu) /6-311+G**//RHF/6-311+G**) and -14.9 kcal/mol (B3LYP//6-311+G**//B3LYP/6-31 1+G**). The aromaticity of 1,2-diseleno-3,4-dithiosquaric acid is indicated by the calculated diamagnetic susceptibility exaltation (A) -17.91 (CSGT(I GAIM)-RHF/6-311+G**//RHF/6-311+G**) and -31.01 (CSGT(IGAIM)-B3LYP/G-311+G** // B3LYP/6-311+G**). Thus, 1,2-diseleno-3,4-dithiosquaric acid fulfils the geometric, energetic and magnetic criteria of aromaticity. The calculated t heoretical gas-phase acidity is DeltaG(1(298K))(o) = 302.7 kcal/mol and Del taG(2(298K))(o) = 388.4 kcal/mol. Hence, 1,2-diseleno-3,4-dithlosquane acid is a stronger acid than squaric acid(3,4-dihydroxy-3-cyclobutene-1,2-dione , H2C4O4).