Microwave Spectrum and Unusual Geometry of Propadienone (Methylene Ketene)

The microwave spectra of several isotopic species of propadienone, CH2=C=C=0, have been analyzed to elucidate the geometry of this somewhat peculiar molecule. The heavy chain of atoms is found to be bent at the middle carbon by approximately 26° from the linear configuration. The nonlinearity is confirmed by the identification of cis and trans forms of monodeuteriopropadienone, by the lack of intensity alternation in the spectral lines, and by a substantial perpendicular component of the dipole moment [pb = 0.7914 (6) D.; µa = 2.156 (3) D.; µtotal, = 2.297 (3) D]. Both the geometry and the dipole moment are at variance with recent results of elaborate molecular orbital calculations.