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

✍ Scribed by Lixin Zhou; Chaoyong Mang; Yongfan Zhang; Shengchang Xiang; Zunxing Huang

• Publisher: John Wiley and Sons
• Year: 2000
• Tongue: English
• Weight: 167 KB
• Volume: 78
• Category: Article
• ISSN: 0020-7608
• DOI: 10.1002/(sici)1097-461x(2000)78:6<443::aid-qua5>3.0.co;2-f

✦ Synopsis

Results of ab initio self-consistent-field and density functional theory calculations of the gas-phase structure, acidity (free energy of deprotonation, G 0 ), and aromaticity of tetrathiosquaric acid (3,4-dithiohydroxy-3-cyclobutene-1,2-dithione, H 2 C 4 S 4 ) 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 relative to reference compounds, cyclobutanedithione, and cyclobutenedithiol. The 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 * * //B3LYP/6-311+G * * ). The aromaticity of tetrathiosquaric acid is indicated by the calculated diamagnetic susceptibility exaltation ( ) -11.77 (CSGT(IGAIM)-RHF/6-311+G * * //RHF/6-311+G * * ) and -18.08 (CSGT(IGAIM)-B3LYP/6-311+G * * //B3LYP/6-311+G * * ). Thus, tetrathiosquaric acid fulfils the geometric, energetic and magnetic criteria of aromaticity. The most reliable theoretical gas-phase acidities are G 0 1(298K) = 303.7 and G 0 2(298K) = 394.1 kcal/mol. Hence, tetrathiosquaric acid is a stronger acid than squaric acid (3,4-dihydroxy-3cyclobutene-1,2-dione, H 2 C 4 O 4 ). Comparisons of the computed results of tetrathiosquaric acid with squaric acid have also been made.