Inelastic tunnelling electrons are a proper excitation source to induce chemical transformations on a single adsorbate. When their energy is tuned to that of molecular vibrational states, the modification may follow complex internal vibrational pathways. Here, we analyse our recent results on the selective excitation of ammonium stretching or bending modes to control the outcome of a simple bond–cleavage reaction. With the help of model calculations, we provide a detailed molecular–scale picture of the competing internal pathways leading to molecular movement. A mode–selective strategy, based on local excitations of specific reaction coordinates, has an important drawback when applied to adsorbate systems due to the problem of fast energy randomization. The success of such a mode–selective strategy is determined here by the ability of the scanning tunnelling microscope to study reactivity in the limit of very low yield and very low power irradiation, in a regime where vibrational heating of the adsorbate/surface system becomes negligible.