In this paper the microscopic dynamics of methyl side groups in polyisoprene is studied by means of inelastic neutron scattering. By combining time-of-flight and backscattering technique a range of four decades can be obtained (0.2 ps-2 ns). The two experimental results were combined in the time domain by using an inverse Fourier transform. Multiple scattering effects were treated by a novel procedure acting on the time-dependent intermediate scattering function S(Q,t). In the description of the data incoherent and coherent scattering from "fixed" atoms was taken into account, i.e., atoms in the main chain that move too slow to be observed in the dynamical window of the experiment. In this way good agreement with the rotation rate distribution model of a threefold jump could be obtained. Seeming discrepancies of the elastic incoherent structure factor vanish after the corrections mentioned above. The distribution of activation energies can be expressed as a Gaussian with an average of 9.7 kJ/mol and a width of 30%. It turns out that the width calculated by the model fit of the data depends on the use of the multiple scattering correction while the average activation energy can also be obtained reliably without that correction from high Q spectra.