An ab initio molecular orbital study of the structure and properties of propadienone (methyleneketene)

Abstract

Ab initio molecular orbital theory with minimal (STO-3G) and split- valence (4-31G) basis sets has been used to obtain fully optimized structures for propadienone, allene, butatriene, carbon dioxide, ketene and vinylidene. Propadienone is predicted to be a planar molecule with C_2V symmetry. The systematic deficiencies of the STO-3G and 4-31G basis sets have been taken into account in deriving a complete r_o structure for propadienone. A striking feature of this structure is the HCH angle (117.°) which is about 5° smaller than the corresponding angle (122.3°) in ketene. An estimate (125 kJ mol^-1) of the heat of formation of propadienone is reported. The alternating dipole moment magnitudes in the series H₂CO, H₂CCO and H₂CCCO are explained in terms of orbital interactions. Propadienone is predicted to be considerably (about 135 kJ mol^-1) more stable than vinylidene+carbon monoxide but slightly (about 10-20 kJ mol^-1) less stable than acetylene+carbon monoxide.