Molecular structure and infrared spectra of 3,4-dihydroxy-3-cyclobutene-1,2-dione; experimental matrix isolation and theoretical Hartree-Fock and post Hartree-Fock study

The infrared spectra of 3,4-dihydroxy-3-cyclobutene-1,2-dione (squaric acid) isolated in low-temperature Ar and N₂ matrices are reported. The comparison of the experimental spectra with those theoretically calculated at HF/6-31 + + G**, MP2/6-31G** and DFT(B3-LYP)/6-31 + +G** levels shows that the molecules of the compound isolated in low-temperature matrix adopt the structure with C_2v symmetry. This structure is different from the one which was found in the crystal. Theoretical calculations of relative energies of C_2v and C_s structures predict the C_2v conformation to be lower in energy by 8 kJ mol^-1. Good agreement between the experimental and theoretical spectra allowed for a reliable assignment of the observed IR absorption bands.