The kinkiness of cumulenones:: H2C3O, H2C4O, and H2C5O

The floppiest bending or "kinking" modes of propadienone (H(2)CCCO), butatrienone (H(2)CCCCO), and pentatetrenone (H(2)CCCCCO) are investigated with high-quality ab initio methods. The potential surface for bending at the beta-carbon atoms of these species is very flat for the C(4) and C(5) species, and-together with zero-point energy contributions from the other vibrations, it is concluded that the barriers to linearity of their carbon chains are less than 1 kJ mol(-1), if they exist at all. Comparisons are made to published experimental values for rotational constants and floppy-mode barrier heights (from gas-phase microwave spectra) and vibrational fundamentals (from matrix infrared spectra). Consensus is achieved for propadienone and butatrienone, and the arguments for infrared detection of synthesized pentatetrenone in an argon matrix are strongly supported by our calculations. [S0021-9606(98)00707-7].