Knowledge Bank

Over 300 rotational transitions of the free radicals HCCCO AND DCCCO between 80 and 400 GHz have been observed by millimeter-wave glow discharge spectroscopy. Analysis of ground state energy levels of $N=8=40$ and $Ka=0-3$ using the S-reduced asymmetric top effective Humiltonian, including treatment of a reasonant spin-rotation perturbation, confirms the qualitative ab initio result that HCCCO and DCCCO have highly prolate bent chain $structures1$. However, the rotational constants are reproduced with better precision by removing one hydrogen from the experimental $H2CCCO$ $structure,2$ implying that HCCCO is more bent than predicted. The rotation and spin-rotation constants indicate that the Renner-Teller effect in this system is stronger than in $HCCCO13$ and the high degree of distortion is evidence for substantial coupling between bending vibration and rotation about the a-axis. Analysis of the moments of inertia of the two isotopomers yield the a-- and b--axis coordinates of the hydrogen atom and a -- and b--axis moments of inertia for the CCCO subunit in the vibrationally averaged structure.