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 pha ses of 3,4-bis(dimethylamino)-3-cyclobutene-1,2-dione and its isomer b is(dimethylamino)squaraine. The crystal structures of the two compound s have been established by XRD and compared with those of other bis(di alkylamino) analogues already studied by the same technique. Ab initio calculations for the squaraine led to a 6-31G* geometry substantiall y similar to the experimental one, indicating that crystal packing onl y marginally perturbs the equilibrium configuration of the isolated mo lecule. In contrast, the geometry of the 3-cyclobutene-1,2-dione deriv ative shows a degree of planarity considerably greater in the crystal than in the free molecule at the MP2/6-31G minimum-energy nuclear con figuration. The squaraine is predicted to be more stable, at all basis set levels here adopted, than its isomer. Differences and similaritie s between experimental IR frequencies and their ab initio estimates ar e presented.