THE ROTATIONAL SPECTRUM AND STRUCTURE OF A WEAKLY-BOUND COMPLEX OF KETENE AND ACETYLENE

Rotational spectra of CH2CO-C2H2, CD2CO-C2H2, CH2CO-C2HD, and CH2CO-C2D2 were observed with a pulsed-beam Fabry-Perot cavity Fourier-transform microwave spectrometer. The b-type transitions were split into four states for CH2CO-C2H2 and CD2CO-C2H2, while two states were assigned for CH2CO-C2HD and CH2CO-C2D2. All states were fit individually to a quartic Watson Hamiltonian. Relative intensity measurements consistent with nuclear spin statistical weights, deuterium hyperfine effects, and the spectral splitting of isotopic species show that the hydrogen nuclei of ketene and acetylene are exchanged by tunneling motions. The electric dipole moment of CH2CO-C2H2 was measured to be mu(a) = 0.227(67) x 10(-30) C m [0.068(20) D] and mu(b) = 4.707(3) x 10(-30) C m [1.411 (1) D]. A planar structure is found for the complex (inertial defect, DELTA = 0.4101 u A2) with a distance of 3.601(1) angstrom between the center of mass of acetylene and the carbonyl carbon of ketene. The preferred geometry obtained from the moment of inertia data corresponds to the molecular axes of ketene and acetylene tilted by approximately 25-degrees from parallel alignment with an acetylenic hydrogen directed toward the oxygen of ketene. This structure differs from the crossed configuration expected for a (2pi(s) + 2pi(a)) cycloaddition reaction.