The internal rotation, or torsion, of a methyl group has been implicated in the acceleration of intramolecular vibrational redistribution (IVR) in numerous experimental studies. In the present work, we investigate its interaction with overall molecular rotation. To isolate the effects of torsion-rotation coupling, a simple two-degree-of-freedom model, including only torsion and three-dimensional rotation, is constructed and its dynamics at j=45 for several energies are studied. Investigation of other values of angular momentum indicate that the results reported are largely independent of j. Two primary effects are observed: (i) a shifting of the stable and unstable axes of rotation due to free methyl torsion, and (ii) a limited degree of weakly chaotic dynamics for trajectories whose torsional energy is near the top of its barrier. Chaos is first observed at the lowest energy at which torsion can surmount its barrier, but then disappears from the system at higher energies. Model toluene exhibits only narrow, high-order nonlinear resonances due to the frequency disparity between torsion and rotation, and these resonances are reduced in number by ''selection rules'' that arise in part from the symmetry of the Hamiltonian. These observations are analyzed in depth and simple, but quantitative, models are proposed to explain the magnitude of the rotational axis shift, the limited extent of chaos, and the resonance selection rules. (C) 1997 American Institute of Physics. [S0021-9606(97)03341-2].