[en] Ion-Polar molecule collisions are studied in the Perturbed Rotational State approach. The basic idea is that the molecular rotation is nearly adiabatically deformed during the collision. The semiclassical, impact-parameter method is adopted. This formulation of the collision problem gives an easy way to calculate the rotational excitation cross sections. Numerical examples are given for the low-energy collision between a proton and a CN molecule. A scaling law is derived so that the numerical results obtained can be used also for many other collision systems. (author)