Knowledge Bank

The microwave spectrum of the $CH2CO---CH2=CH2$ complex was observed in a pulsed-beam Fourier transform microwave spectrometer. In addition to the normal isotopic species, which exhibits a b-type rotational spectrum and four states due to internal motions, complexes of ketene with trans-CHD$=$CDH and $CD2=CH2$ have been observed and these also exhibit four states, while the ketene---cis-CHD$=$CHD species exhibits two states. Spectral analysis yields the rotational constants for $CH2CO---CH2=CH2:$ $A=7494.141(2)MHz,B=2276.441(6)MHz,$ and $C=1966.992(6)MHz.$ The structural analysis, assuming fixed monomer geometries, show that the ketene unit lies in the a,b-plane of the complex and the symmetry axis of ethylene is parallel to the c-axis, i.e. the symmetry axis of the two units are crossed at $900$, with a center of mass separation, $Rcm$+ of 3.46 .{A}. Stark effect measurements for the normal isotopic species provide the dipole moment components $ \mu a = 0$ D, $ \mu_{b} = 1.376(1)$ D and $ \mu_{c} = 0$ D which are consistent with the structural results. Further details concerning the tunneling motions and structure analysis will be given. Comparisons with related complexes, such as ketene--acetylene, and cycloaddition prototype species will be presented.