Knowledge Bank

The rotational spectrum of the oblate asymmetric top van der Waals trimer, $Ar2$-ketene, was obtained with a pulsed-beam Fourier-transform microwave spectrometer. Two tunnelling states arising from internal rotation of ketene were assigned for $Ar2CH2CO$ and $Ar2CD2CO$. The least squares fit of the observed $\mu b$ and $\mu c$-type transitions of the ground state normal isotopomer to a semirigid Watson Hamiltonian gave rotational constants of $A=1652.28376(17)MHz.B=1484.42510(16)MHz$ and $C=914.02182(37)MHz$. Electric dipole moment components of $\mu b=0.175(18)D$ and $\mu c=1.343(3)D$ were calculated from Stark shift measurements of the ground state normal isotopomer. The spectral data are consistent with a trimer structure in which the CCO axis of ketene is perpendicular to the Ar-Ar axis. Least squares fits of the moments of inertia of the two isotopomers gave an Ar-Ar separation of 1.916(2){AA}, a ketene center of mass to the Ar-Ar bisector of 3.039(5){\AA} and an angle of $99.5(1.5)\circ$ between the methylene carbon, center of mass of ketene and the point at the bisector of the Ar-Ar separation.