Microwave and infrared measurements are reported for the van der Waals
complex Ar-CH3CCH and its isotopomers. The structure is T shaped with
equilibrium center-of-mass separation of 3.73 angstrom and an angle o
f 82-degrees between the molecule symmetry axis and the van der Waals
bond. The infrared and microwave spectra are complex due to the effect
s of a slightly hindered internal rotor. Analysis of the spectral data
shows that the dipole moment is almost parallel to the dimer b axis.
A modified vibration/rotation Hamiltonian that includes an internal ro
tor potential is used to show that the barrier to internal rotation is
near 10.8 cm-1.