<i>Ab initio</i> and density functional theory studies of the structure, gas‐phase acidity and aromaticity of tetraselenosquaric acid

Abstract

Abstract Results of ab initio self‐consistent‐field (SCF) and density functional theory (DFT) calculations of the gas‐phase structure, acidity (free energy of deprotonation, Δ G *) and aromaticity of tetraselenosquaric acid (3, 4‐diselenyl‐3‐cyclobutene‐1,2‐diselenone, H 2 C 4 Se 4 )are reported. The global minimum found on the potential energy surface of tetraselenosquaric 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 dose in energy. The optimized geometric parameters exhibit a bond length equalization relative to reference compounds, cyclobutanediselenone, and cyclobutanediselenol. 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 acid is indicated by the calculated diamagnetic susceptibility exaltation (A) ‐19.13 (CSGT(IGAIM)‐RHF/6–311 + G**// RHF/6–311 + G** and ‐32.91 (4π·10 − 6 m −3 /mol)(CSGT(I‐GAIM)‐B3LYP/6 ‐ 311 + G* * //B3LYP/6 ‐ 311 + G**). Thus, tetraselenosquaric acid fulfils the geometric, energetic and magnetic criteria of aromaticity. The calculated gas‐phase acidity is Δ G 1 * (298k) = 1257.7 and Δ G * 2 (298k) = 1617.1 kJ/mol. Hence, tetraselenosquaric acid is the strongest acid among the three squaric acids (3, 4‐dihydroxy‐3‐cyclobutene‐1, 2‐dione, H 2 C 4 3,4‐dithiohydroxy‐3‐cyclobutene‐1,2‐dithione, H 2 C 4 S 4 , 3, 4‐diselenyl‐3‐cyclobutene‐1,2‐diselenone, H 2 C 4 Se 4 ).