Geometry and molecular parameters of 3,4-bis(dimethylamino)-3-cyclobutene-1,2-dione and its isomer bis(dimethylamino)squaraine. Combined study by IR spectroscopy, XRD and ab initio MO calculations

Absorption FTIR spectra have been recorded for solutions and solid phases of 3,4-bis(dimethylamino)-3-cyclobutene-1,2-dione and its isomer bis(dimethylamino)squaraine. The crystal structures of the two compounds have been established by XRD and compared with those of other bis(dialkylamino) analogues already studied by the same technique. Ab initio calculations for the squaraine led to a 6–31G** geometry substantially similar to the experimental one, indicating that crystal packing only marginally perturbs the equilibrium configuration of the isolated molecule. In contrast, the geometry of the 3-cyclobutene-1,2-dione derivative shows a degree of planarity considerably greater in the crystal than in the free molecule at the MP2/6–31G* minimum-energy nuclear configuration. The squaraine is predicted to be more stable, at all basis set levels here adopted, than its isomer. Differences and similarities between experimental IR frequencies and their ab initio estimates are presented.