Rotational spectra of the weakly bound complexes Ar-ethylene and Ne-ethylene were measured with a pulsed molecular beam Fourier transform microwave spectrometer in the range from 3.5 to 26 GHz. Spectra of five isotopomers of Ar-ethylene, namely Ar-C2H4, Ar-(C2H4)-C-13, Ar-C2D4, Ar-trans-1,2-C2D2H2, and Ar-cis-1,2-C2D2H2, and of eight isotopomers of Ne-ethylene, namely Ne-20-C2H4, Ne-20-C2D4, Ne-20-trans-1,2-C2D2H2, Ne-20-cis-1,2-C2D2H2, Ne-22-C2H4, Ne-22-C2D4, Ne-22-trans-1,2-C2D2H2, and Ne-22-cis-1,2-C2D2H2, were assigned and analyzed. The spectra are in accord with T-shaped, planar structures, where the rage gas atoms are located on the b-principal inertial axis of the ethylene monomer. For isotopomers containing C2H4, (C2H4)-C-13, C2D4, and trans-1,2-C2D2H2, all observed transitions are doubled due to an internal rotation motion of the ethylene subunit within the complexes. The observed transition intensities are in agreement with nuclear spin statistical weights obtained from molecular symmetry group analyses under the assumption of an internal rotation of the ethylene unit about the C=C bond, i.e., the out-of-plane motion. The observation of K-a=1, m=0 transitions in Ne-trans-1,2-C2D2H2 provides further proof that the out-of-plane motion is responsible for the observed tunneling splittings. Information about the energy level ordering of the K-a=1, m=0 and K-a=0, m=1 states was obtained from the rotational spectra of the Ne-trans-1,2-C2D2H2 isotopomers. Electronic structure calculations of Ne-C2H4 were done at the CCSD(T) level of theory with the aug-cc-pVTZ basis set for all atoms, supplemented with bond functions. The global minimum is at the T-shaped, planar configuration, with a distance of R=3.55 A between the Ne atom and the center-of-mass of ethylene and a well depth of -81.5 cm-1. One-dimensional minimum potential energy paths for possible internal rotation motions were determined. The results confirm that the out-of-plane motion is the preferred internal motion. The out-of-plane minimum potential energy path was used to determine the energy difference between the two lowest tunneling states using the one-dimensional flexible model by Meyer [R. Meyer, J. Mol. Spectrosc. 76, 266 (1979)]. (C) 2003 American Institute of Physics.