2. 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 Bruno Lunelli,Pietro Roversi,Emanuele Ortoleva,Riccardo Destro J. Chem. Soc. Faraday Trans. 1996 92 3611