The Rotational Spectra, Structure, Internal Dynamics, and Electric Dipole Moment of the Argon–Ketene van der Waals Complex

Abstract

Pulsed-beam Fourier transform microwave spectroscopy was used to observe and assign the rotational spectra of the argon–ketene van der Waals complex. Tunneling of the hydrogen or deuterium atoms splits the a- and b-type rotational transitions of H2CCO–Ar, H213CCO–Ar, H2C13CO–Ar, and D2CCO–Ar into two states. This internal motion appears to be quenched for HDCCO–Ar where only one state is observed. The spectra of all isotopomers were satisfactorily fit to a Watson asymmetric top Hamiltonian which gave A=10 447.9248(10) MHz, B=1918.0138(16) MHz, C=1606.7642(15) MHz, ΔJ=16.0856(70) kHz, ΔJK=274.779(64) kHz, ΔK=−152.24(23) kHz, δJ=2.5313(18) kHz, δK=209.85(82) kHz, and hK=1.562(64) kHz for the A1 state of H2CCO–Ar. Electric dipole moment measurements determined μa=0.417(10)×10−30 C m [0.125(3) D] and μb=4.566(7)×10−30 C m [1.369(2) D] along the a and b principal axes of the A1 state of the normal isotopomer. A least squares fit of principal moments of inertia, Ia and Ic, of H2CCO–Ar, H213CCO–Ar, and H2C13CO–Ar for the A1 states give the argon–ketene center of mass separation, Rcm=3.5868(3) Å, and the angle between the line connecting argon with the center of mass of ketene and the C=C=O axis, θcm=96.4°(2). The spectral data are consistent with a planar geometry with the argon atom tilted toward the carbonyl carbon of ketene by 6.4° from a T-shaped configuration.