During a recent investigation of the photophysical properties of Ru(II) complexes containing the ligand 4,4′-bipyridine we had occasion to prepare an intermediate, 1,4-bisethoxymethylene-2,3-butadione, abbreviated as EMB. We were surprised to find a strong band at ca. 1590cm −1 (IR spectra) which had all the characteristics of a carbonyl stretch but saw no comparable bands in the region where one would normally expect the CO resonance. This frequency is the lowest among all known organic carbonyl compounds. We performed theoretical calculations on isolated EMB molecule by using Gaussian 94 package (RHF and DFT methods with the 6-311G and 6-311+G(d,p) basis sets). Within the DFT methods the functionals SVWN and B3LYP have been tested. Calculated IR, Raman, 1H NMR and 13C NMR spectra appeared to be in good agreement with experimental data. We also calculated potential energy distribution (PED) for presented in the paper vibrational bands.