Ab initio and density functional theory studies of the structure,gas-phase acidity and aromaticity of tetraselenosquaric acid

Results of ab initio self-consistent-field (SCF) and density functional theory (DFT) calculations of the gas-phase structure,acidity (free energy of deprotonation,G0) and aro-maticity of tetraselenosquaric acid (3,4-diselenyl-3-cy-dobutene-1,2-diselenone,H2C4Se4) are reported.The global minimum found on the potential energy surface of tetraselenosquaric acid presents a planar conformation.The ZZ iso-mer was found to have the lowest energy among the three planar conformers and the ZZ and ZE isomers are very dose in energy.The optimized geometric parameters exhibit a bond length equalization relative to reference compounds,cyclobu-tanediselenone,and cydobutenediselenol.The computed aromatic stabilization energy (ASE) by homodesmotic reaction is -77.4 (MP2(fu)/6 - 311 G //RHF/6 - 311 G) and - 54.8 kJ/mol (B3LYP/6 - 311 G //B3LYP/6 -311 G).The aromaticity of tetraselenosquaric add is indicated by the calculated diamagnetic susceptibility exaltation (A) - 19.13 (CSGT(IGAEM) - RHF/6 - 311 G// RHF/6-311 G and -32.91 (4 10-6 m3/mol)(CSGT(I-GAM)-B3LYP/6 - 311 G//B3LYP/6 - 311 G).Thus,tetraselenosquaric add fulfils the geometric,energetic and magnetic criteria of aroniaticity.The calculated gas-phase acidity is AG10=1257.7 and G2=1617.1 kJ/mol.Hence,tetraselenosquaric add is the strongest add among the three squaric acids (3,4-dihydroxy-3-cyclobutene-1,2-dione,H2C4O4,3,4-dithiohydroxy-3-cydobutene-1,2-dithione,H2C4S4,3,4-diselenyl-3-cydobutene-1,2-diselenone,H2C4Se4)