Evidence for heavy atom large amplitude motions in RG-cyclopropane van der Waals complexes (RG=Ne, Ar, Kr) from rotation-tunneling spectroscopy

Rotation-tunneling spectra of the van der Waals complex Ne-cyclopropane, and pure rotational spectra of Ar-cyclopropane and Kr-cyclopropane, were measured with a Fourier transform microwave spectrometer in the frequency range from 4-19 GHz. The observed transitions are all of a-type and are in accord with those of prolate symmetric top molecules where the rare gas atom is positioned on the C-3 axis of the cyclopropane subunit above its symmetry plane. Altogether transitions of five isotopomers of the Ne complex with J values ranging from 0 to 4 and K values from 0 to 1 were measured. Three isotopomers of the Ar complex (J from 1 to 5 and K from 0 to 2) and nine isotopomers of the Kr complex (J from 1 to 8 and K from 0 to 2) were observed. Effective separations between the center-of-mass of cyclopropane and the rare gas atoms were obtained from the ground state rotational constants and were found to be 3.673, 3.802, and 3.906 Angstrom for the Ne-, Ar-, and Kr-containing complexes, respectively. The derived centrifugal distortion constants indicate that the complexes are comparatively rigid in the radial coordinate. Transitions of two isomers were observed for complexes that contain monodeuterated cyclopropane. The intensities of transitions of the D-bonded species were considerably higher compared to those of the H-bonded isomers indicating that the angular mellon of the rare gas atom is much less restricted. In the case of the parent Ne-cyclopropane complex, and its Ne-22- and mono C-13-isotomers, all transitions appeared as doublers, This is attributed to a tunneling internal rotation motion of the cyclopropane unit within the complex. (C) 1997 American Institute of Physics.