Are cumulenones kinked? A systematic high-level ab initio study of H₂CCCO, H₂CCCCO and H₂CCCCCO

The cumulenones, propadienone (H₂C=C=C=O), butatrienone (H₂C=C=C=C=O) and pentatetraenone (H₂C=C=C=C=C=O) have been systematically investigated with high-quality ab initio methods. At our highest levels of theory, the equilibrium structures of all three cumulenones show significant bending in the heavy-atom chain. Although the barrier to linearity of the carbon chain is significant for propadienone (approximately 5 kJ mol^-1), the calculated barriers for butatrienone and pentatetraenone are very small (less than 0.5 kJ mol^-1). Calculated spectroscopic properties of the cumulenones are in good agreement with available experimental data.