THE ROTATIONAL SPECTRUM OF KETENE DIMER

A pulsed-beam Fourier-transform microwave spectrometer was used to investigate the rotational spectrum of the van der Waals dimer of ketene. Five states arising from internal motions of ketene were assigned for the normal isotopomer. Only one state could be fit satisfactorily to an asymmetric top Watson Hamiltonian. The least squares fit of the observed $\\mu_{a}, \\mu_{b}$ and $\\mu_{c}$-type transitions gave rotational constants of $A = 5263.2758(21)MHz. B = 1826.4985(12)MHz$ and $C = 1642.7707(12)MHz$. Electric dipole moment components of $\\mu_{a} = 1.527(4)D, \\mu_{b} = 0.654(23)D$ and $\\mu_{c} = 2.655(9)D$ were calculated from Stark shift measurements. The spectral data are consistent with a head to head crossed dimer structure and least squares fits of the moments of inertia give a ketene separation of 3.359(1){\\AA}. The structure is in agreement with a $2 \\pi_{***}+2\\pi_{***}$ approach of two ketene monomers as predicted by orbital symmetry analysis for the observed dimerization reaction of ketene.