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 N-2 matrices are reported. T he comparison of the experimental spectra with those theoretically cal culated at HF/6-31 + + G*, MP2/6-31G** and DFT(B3-LYP)/6-31++G** leve ls shows that the molecules of the compound isolated in low-temperatur e matrix adopt the structure with C-2v symmetry. This structure is dif ferent from the one which was found in the crystal. Theoretical calcul ations of relative energies of C-2v and C-s structures predict the C-2 v conformation to be lower in energy by 8 kJ mol(-1). Good agreement b etween the experimental and theoretical spectra allowed for a reliable assignment of the observed IR absorption bands. (C) 1997 Elsevier Sci ence B.V.