Ab initio molecular orbital calculations at the MP2/6-31G* level have been used to examine the structures and infrared spectra of a new class of compounds, the iminopropadienones, RN=C=C=C=O (R = H, CH3, and Ph). The agreement between calculated and experimental IR spectra of PhNCCCO, Ph(15)NCCCO, and PhNCC(13)CO is excellent. Inclusion of electron correlation is found to be important for the correct prediction of the relative intensities of the cumulenic stretching vibrations. All three iminopropadienones are predicted to have a slightly bent NCCCO backbone (angle CCC approximate to 176 degrees). As with carbon suboxide, these cumulenes are calculated to have an extremely flat CCC bending potential. The parent compound, HNCCCO (4a) is calculated to be thermodynamically stable toward dissociations into (i) HNCC + CO, (ii) HNC + CCO, (iii) HN + CCCO, and (iv) H. + NCCCO.. Rearrangement of 4a to the more stable cyanoketene isomer requires a sizeable barrier of 402 kJ mol(-1) [G2(MP2)]. The calculated stability of 4a is consistent with its experimental observation in neutralization-reionization mass spectrum. The adiabatic ionization energy and heat of formation of HNCCCO are predicted to be 9.51 eV and 175 kJ mol(-1), respectively.