The gas-phase acidity and aromaticity of squaric acid: an ab initio and density functional theory study

Abstract

The conformation, aromaticity and gas-phase acidity (free energy of deprotonation, ΔGo) of squaric acid (3,4-dihydroxy-3-cyclobutene-1,2-dione) were calculated at the SCF and MPn (n=2,3,4), and B3LYP levels using the 6-311G(d,p) and 6-311+G(d,p) basis sets. The global minimum found on the potential energy surface of squaric acid presents a planar conformation. The ZZ isomer was found to be the most stable of the three planar conformers and the ZZ and ZE isomer are very close in energy. The optimized geometrical parameters exhibit a bond length equalization relative to reference molecules, cyclobutandione, and cyclobutenediol. The computed aromatic stabilization energy (ASE) by homodesmotic reaction is −16.3 (MP4(sdtq)/6-311+G(d,p)//RHF/6-311+G(d,p)) and −12.9kcal/mol (B3LYP/6-311+G(d,p)//B3LYP/6-311+G(d,p)). The aromaticity of squaric acid is indicated by the calculated diamagnetic susceptibility exaltation (Λ)−2.07 (CSGT(IGAIM)-RHF/6-311+G(d,p)//RHF/6-311+G(d,p)) and −2.16 (CSGT(IGAIM)-B3LYP/6-311+G(d,p)//B3LYP/6-311+G(d,p)). Thus, squaric acid fulfils the geometrical, energetic and magnetic criteria of aromaticity. The most reliable theoretical gas-phase acidity is ΔG1(298K)o=311.4 and ΔG2(298K)o=421.0kcal/mol.